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/api/basic_json/contains.md

@@ -61,11 +61,18 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 ## Complexity
 
-Logarithmic in the size of the JSON object.
+1. Logarithmic in the size of the JSON object.
+2. Logarithmic in the size of the JSON object.
+3. At most one lookup per reference token of `ptr`, each logarithmic in the size of the container at that level:
+   O(*n* log *m*) for *n* reference tokens and containers of size at most *m*.
+4. Deleted; a call with an integral argument does not compile.
 
 ## Notes
 
-- This method always returns `#!cpp false` when executed on a JSON type that is not an object.
+- Overloads (1) and (2) always return `#!cpp false` when executed on a JSON type that is not an object. Overload (3)
+  also works on arrays and on primitive values: the empty JSON pointer resolves in every JSON value (so
+  `#!cpp json(1).contains(""_json_pointer)` returns `#!cpp true`), and `#!cpp json({1, 2}).contains("/1"_json_pointer)`
+  returns `#!cpp true`.
 - This method can be executed on any JSON value type.
 - Calling this function with an integer argument (for example, `#!cpp contains(0)`) does not compile: such an argument
   would otherwise implicitly convert to a null `#!cpp const char*` and, from there, cause undefined behavior when