Return the first member of duplicate keys from lookups again

Looking up the last member of a duplicate key cannot stop at a match, so
every lookup scanned the whole object (1.6 to 3.4 times slower for small
objects). operator[](key), at, find, value, contains, count, and JSON
pointer resolution return the first member again, as yyjson and simdjson
do; materialize() and get<map>() keep the last value, as parse().

The documentation says so in the feature page and on each lookup page,
and explains how to get the value parse() would give. The integer index
templates and the discarded chaining of operator[] stay.

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

docs/mkdocs/docs/api/basic_json_view/operator[].md

@@ -15,7 +15,7 @@ basic_json_view operator[](IntegerType idx) const;
 basic_json_view operator[](const json_pointer& ptr) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
    the [Notes](#notes) below) -- or a [discarded](is_discarded.md) view if there is no such member.
 2. Returns the array element at index `idx`, or a [discarded](is_discarded.md) view if `idx` is out of range. The
    template accepts every integer type except `#!cpp bool` and `#!cpp std::size_t` (`#!cpp int`, `#!cpp unsigned`,
@@ -38,7 +38,7 @@ basic_json_view operator[](const json_pointer& ptr) const;
 
 ## Return value
 
-1. the value of the last member with key `key`, or a discarded view if no member has this key (or if this view is
+1. the value of the first member with key `key`, or a discarded view if no member has this key (or if this view is
    [discarded](is_discarded.md))
 2. the element at index `idx`, or a discarded view if `#!cpp idx >= size()` or `idx` is negative (or if this view is
    [discarded](is_discarded.md))
@@ -80,7 +80,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Complexity
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   another, in document order, stopping at the first match. Each comparison first checks the
    key's length -- already known from the index, without reading the key bytes -- before comparing its content, so a
    key of a different length than `key` is rejected without touching the source text.
 2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
@@ -105,15 +105,18 @@ document.
     key on an array or a primitive, or an index on an object or a primitive, is `type_error.305` as for
     `BasicJsonType`. [`at`](at.md) still throws for a discarded view, as it does for a missing key.
 
-!!! info "Duplicate keys"
+!!! warning "Duplicate keys: the first member wins"
 
     If the source text has an object with a duplicate key, `#!cpp operator[]` (and [`at`](at.md), [`find`](find.md),
     [`contains`](contains.md), [`count`](count.md), [`value`](value.md), and JSON pointer resolution) all resolve to
-    the *last* member with that key. This is the member [`materialize()`](materialize.md) (and
-    [`BasicJsonType::parse()`](../basic_json/parse.md)) keeps, so a lookup in the view and in the materialized value
-    agree. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
-    duplicates, in document order. A lookup scans all members for this: it cannot stop at the first match. See the
-    example below and [`size()`](size.md#notes).
+    the *first* member with that key, because a lookup can stop as soon as it finds a match. This is different from
+    [`materialize()`](materialize.md) (and [`BasicJsonType::parse()`](../basic_json/parse.md)), which replay every
+    member in order and so keep the *last* value for a repeated key, so `#!cpp v["a"]` and
+    `#!cpp v.materialize()["a"]` can differ. To get the value `parse()` would give, use
+    [`materialize()`](materialize.md) or iterate the members with [`items()`](items.md) and keep the last match.
+    [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
+    duplicates, in document order. See [Duplicate keys](../../features/json_view.md#duplicate-keys) and
+    [`size()`](size.md#notes).
 
 !!! info "JSON pointer resolution"