Remove the library-wide -Wdocumentation pragma; fix what it hid

macro_scope.hpp / macro_unscope.hpp pushed and popped a Clang
diagnostic region over the entire library that ignored -Wdocumentation
and -Wdocumentation-unknown-command. Removed both pragmas and fixed
every finding a full -Wdocumentation (which implies
-Wdocumentation-unknown-command and -Wdocumentation-deprecated-sync)
build reports, so the library now compiles clean under Clang's
documentation checks without a blanket suppression. Overlaps #5267,
which is still open and edits a nearby doc block (json.hpp's
get()/get_impl() @return, already fixed in the item 2 step (b) commit
of this branch); this commit does not touch that block again.

Unknown Doxygen alias commands (Doxyfile removed in #3071, so these
were never rendered by anything) rewritten as plain prose, keeping the
same information:
- @requirement REQ-JSON-01 / REQ-JSON-02 (iter_impl.hpp,
  json_reverse_iterator.hpp): now "This class satisfies the following
  concept requirements (REQ-JSON-0N):".
- @liveexample{prose,example-id} (three sites in json.hpp): kept the
  prose, dropped the command wrapper and the trailing example-id
  (docs/mkdocs/docs/examples/*.cpp still exist and are used directly
  by the rendered docs, not through this in-header alias) and
  unescaped the "\," commas that were only needed for the old alias's
  comma-separated argument syntax.
- @complexity X (json.hpp x4, json_pointer.hpp x2, serializer.hpp x1):
  now "Complexity: X".

Backslash sequences Clang's comment lexer tried to parse as commands,
escaped to render as literal backslashes:
- lexer.hpp get_codepoint(): two `\u` occurrences.
- binary_reader.hpp get_bson_cstr() / get_bson_cstr_bulk(): two
  `\x00` occurrences.
- serializer.hpp: three `\uXXXX` occurrences (constructor @param,
  append_codepoint_to_string_buffer() @brief, and the ensure_ascii
  member comment).

One finding remained after all of the above: Clang reports
"declaration is marked with '@deprecated' command but does not have a
deprecation attribute" on the deprecated sax_parse(span_input_adapter&&, ...)
overload, even though JSON_HEDLEY_DEPRECATED_FOR does expand to
__attribute__((deprecated(...))) for Clang. Several isolated
reproductions of this exact declaration shape - doc comment,
template<>, two stacked __attribute__ macros, an overload set sharing
the name - did not reproduce the warning, so this looks like a
Clang comment/declaration-association quirk specific to this overload
inside the much larger basic_json class template, not an actual
documentation defect. Rather than keep the pragma library-wide for one
Clang false positive, added a tightly scoped
-Wdocumentation-deprecated-sync push/pop around just that overload.

Verified with Apple clang 21 and the project's actual -Weverything
flag set (cmake/clang_flags.cmake) on the full header at -std=c++11
and -std=c++20: zero -Wdocumentation* diagnostics. Also compiled
clean with GCC 16.2.0 (the pragmas are already __clang__-gated, so
this only confirms no unrelated breakage). Ran make check-amalgamation
and the full local test suite: 129/129 passing.

#5725 item 2 (step c)

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

include/nlohmann/detail/input/binary_reader.hpp

@@ -407,7 +407,7 @@ class binary_reader
     @brief Parses a C-style string from the BSON input.
     @param[in,out] result  A reference to the string variable where the read
                             string is to be stored.
-    @return `true` if the \x00-byte indicating the end of the string was
+    @return `true` if the \\x00-byte indicating the end of the string was
              encountered before the EOF; false` indicates an unexpected EOF.
     */
     bool get_bson_cstr(string_t& result)
@@ -437,7 +437,7 @@ class binary_reader
     @brief read a C-style string from contiguous input in one step
 
     @param[in,out] result  the string to append to
-    @return whether the string was read; if the input has no \x00-byte, nothing
+    @return whether the string was read; if the input has no \\x00-byte, nothing
             is read, and @ref get_bson_cstr reports the end of the input
     */
     bool get_bson_cstr_bulk(string_t& result, std::true_type /*bulk*/)