docs: correct JSON Pointer documentation and add tests (#5937)

Fix inaccurate or missing statements about unflatten (third round-trip
failure, empty token, typo), value (type_error.306 before pointer lookup),
contains (pointer overload), parent_pointer and operator<< (bad_alloc),
out_of_range.410 example, template parameter requirements (resize(n)), and
document the URI fragment form and non-unique member names of RFC 6901.
Add tests for the unflatten and value() cases.

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

docs/mkdocs/docs/features/json_pointer.md

@@ -37,6 +37,10 @@ Then every value inside the JSON document can be identified as follows:
 Note `/` does not identify the root (i.e., the whole document), but an object entry with empty key `""`. See
 [RFC 6901](https://tools.ietf.org/html/rfc6901) for more information.
 
+[RFC 6901, section 4](https://tools.ietf.org/html/rfc6901#section-4) states that evaluation fails if an object has
+non-unique member names. A `basic_json` object holds one value per key (see [`object_t`](../api/basic_json/object_t.md)
+for how duplicate keys are handled while parsing), so evaluating a JSON Pointer never encounters duplicates.
+
 ## JSON Pointer creation
 
 JSON Pointers can be created from a string:
@@ -54,6 +58,13 @@ auto p = "/nested/one"_json_pointer;
 The escaping rules of [RFC 6901](https://tools.ietf.org/html/rfc6901) are implemented. See the
 [constructor documentation](../api/json_pointer/json_pointer.md) for more information.
 
+!!! note "URI fragment representation"
+
+    [RFC 6901, section 6](https://tools.ietf.org/html/rfc6901#section-6) also defines a URI fragment identifier
+    representation (`#/a%20b`). It is not supported: `#!cpp json::json_pointer("#/a")` throws
+    [`parse_error.107`](../home/exceptions.md#jsonexceptionparse_error107). Strip the leading `#` and
+    percent-decode the remainder before creating the JSON pointer.
+
 ## Value access
 
 JSON Pointers can be used in the [`at`](../api/basic_json/at.md), [`operator[]`](../api/basic_json/operator%5B%5D.md),