Address the rest of the review of editable json documents

From gregmarr's review of #5630:

- alloc_nodes() gives a request of half of the next chunk or more a chunk
  of its own, so that the rest of the current chunk stays available for
  smaller requests instead of being abandoned.
- The comments on edit_state, append_text(), and reserve_moved() say that
  views stay valid across every edit (not just the first), that n is 0 for
  empty strings, and why both capacities are checked.
- The architecture page names flag bits 3, 4, and 5 (linked) separately
  and says that a discarded view has no node; erase.md rewords overload 3
  and says why it never throws invalid_iterator.202.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_document/erase.md

@@ -18,10 +18,10 @@ has `erase`; calling it on a read-only `basic_json_document` fails to compile (`
 1. Removes every member of `object` whose key is `key` (see [Notes](#notes) on duplicate keys) and returns how many
    were removed; `#!cpp 0` if `object` has no member with this key.
 2. Removes the element at index `idx` of `array`, which must already exist (`#!cpp idx < array.size()`).
-3. Removes the value the JSON pointer `ptr` refers to, relative to [`root()`](root.md), and returns how many values
-   were removed: the *parent* of the target must already exist, and the target itself is removed as in 1. (an object
-   member; `#!cpp 0` or more) or 2. (an array element; always `#!cpp 1`). `ptr` must not be empty -- [`root()`](root.md)
-   itself cannot be erased.
+3. Removes the value the JSON pointer `ptr` refers to, relative to [`root()`](root.md). `ptr` must not be empty
+   ([`root()`](root.md) itself cannot be erased), and the *parent* of the target must already exist. The target is
+   removed as in 1. if the parent is an object (returns the number of removed members, `#!cpp 0` or more) and as in 2.
+   if it is an array (always returns `#!cpp 1`).
 
 ## Template parameters
 
@@ -71,8 +71,8 @@ has `erase`; calling it on a read-only `basic_json_document` fails to compile (`
 
 Overloads 1. and 2. also throw [`invalid_iterator.202`](../../home/exceptions.md#jsonexceptioninvalid_iterator202)
 ("view does not belong to this document") if `object`/`array` is a [discarded](../basic_json_view/is_discarded.md)
-view or a view of a *different* document. Overload 3. takes no view: it always starts from this document's own
-[`root()`](root.md).
+view or a view of a *different* document. Overload 3. never throws `invalid_iterator.202`: it takes no view, and it
+always starts from this document's own [`root()`](root.md).
 
 ## Complexity
 

docs/mkdocs/docs/home/architecture.md

@@ -194,8 +194,8 @@ packet-beta
 
 | Bytes | Field   | Type       | Contents                                                                                                                      |
 |-------|---------|------------|-------------------------------------------------------------------------------------------------------------------------------|
-| 0     | `kind`  | `uint8_t`  | the type, numbered as [`value_t`](../api/basic_json/value_t.md): 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float; 10 for a link (see below) |
-| 1     | `flags` | `uint8_t`  | bits 0-1: where a string's bytes (or a number's token) are (0: the source text, 1: the buffer of decoded strings, for strings with escapes, 2: the edit buffer); bit 2: the value of a boolean; bits 3 and 4: moved and new (see below) |
+| 0     | `kind`  | `uint8_t`  | the type, numbered as [`value_t`](../api/basic_json/value_t.md): 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float; 10 for a link (see below). There is no node for a discarded value: a [discarded](../api/basic_json_view/is_discarded.md) view has none |
+| 1     | `flags` | `uint8_t`  | bits 0-1: where a string's bytes (or a number's token) are (0: the source text, 1: the buffer of decoded strings, for strings with escapes, 2: the edit buffer); bit 2: the value of a boolean; bit 3: moved, bit 4: new, bit 5: linked (see below) |
 | 2-3   | `extra` | `uint16_t` | numbers: the number of integer digits (low byte) and fraction digits (high byte), 255 for more; objects: the number of their key hash index (1-based), or 0; otherwise 0 |
 | 4-7   | `off`   | `uint32_t` | where the value starts: the first byte after a string's opening quote (or its position in the buffer of decoded strings), the first byte of a number or literal, the bracket of an array or object |
 | 8-11  | `len`   | `uint32_t` | strings: the length after decoding; floats and literals: the length of the token; arrays and objects: the number of elements |
@@ -257,6 +257,8 @@ edited:
   sequence (a header node, then the entries), whose number is in `off`. The entries are links (`kind` 10), whose bytes
   8-15 hold the address of the value's node, so values never move.
 - A node written by an edit gets the flag *new* (bit 4): it has no position in the source text.
+- A value that an entry of such a sequence links to gets the flag *linked* (bit 5): its extent in the parsed layout
+  (`next`) no longer matters, so a scalar can replace an array or object there in place.
 - Views of read-only documents compile without any of this: how views walk the index is a template parameter
   (`navigation<Editable>`).