Address the review of json_view element access

From gregmarr's review of #5622:

- count() counts every member with the key, so count(key) > 1 detects
  duplicate keys; it was contains() as 0/1 before.
- get_to() passes types that get<T>() does not convert directly to
  from_json with the existing value, as basic_json::get_to() does, so that
  a from_json which sets only the fields present keeps the others.
- array_index() checks the empty token and "-" first; the results are
  unchanged.
- Docs: no "if an exception is thrown" where a view never changes
  anything; the borrowed source must also stay unmodified; items() on a
  temporary view is safe; back()/front() notes shortened; the basic_json
  see-also entries say how the view's results can differ.

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

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

@@ -50,7 +50,7 @@ basic_json_view operator[](const json_pointer& ptr) const;
 
 ## Exception safety
 
-Strong exception safety: if an exception is thrown, there are no changes to the view or the document it refers to.
+Strong guarantee: there are no changes to the view or the document it refers to.
 
 ## Exceptions
 
@@ -108,7 +108,7 @@ document.
 !!! 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
+    [`contains`](contains.md), [`value`](value.md), and JSON pointer resolution) all resolve to
     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