Document the contract of object keys passed to json_sax::key and key callbacks (#5956)

* Document the contract of object keys passed to json_sax::key and key callbacks

Describe what the name passed to json_sax::key and to a parser callback
for parse_event_t::key holds when parsing JSON text: escapes are decoded
to UTF-8, different spellings of a name yield identical strings, the name
is validated before the call (invalid escapes or UTF-8 give a parse error
and no key event), no normalization or case conversion is applied, the
event precedes the ':' check, and duplicate names are reported once per
occurrence. Also note that sax_parse passes binary-format keys unchanged.

See discussion #5955.

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

* Add a test for the object key contract and correct the BON8 note

Pin the guarantees documented for json_sax::key and the key event of
parser callbacks: decoded names, validation before delivery, delivery
before the colon, one event per repeated name, and the binary formats.
BON8 rejects ill-formed UTF-8 in a name even with sax_parse, so the
note no longer lists it with the formats that pass bytes through.

See discussion #5955.

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

* Fix the GCC build of the object key contract test

GCC 11 in C++11 mode rejects a default argument on a lambda parameter
(-Wc++14-extensions) and ignores a static_cast<void> on the
warn_unused_result of parse(). Pass the input format explicitly and bind
the parse result to a variable.

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

* Add version history for the object key contract

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

---------

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

docs/mkdocs/docs/features/parsing/sax_interface.md

@@ -64,6 +64,9 @@ bool parse_error(std::size_t position, const std::string& last_token, const json
 
 The return value of each function determines whether parsing should proceed.
 
+Object keys are passed to `key` decoded and validated: escape sequences are already resolved, so different spellings of
+the same name yield the same string. See [`key`](../../api/json_sax/key.md#notes) for the exact guarantees.
+
 ## Stepping through the events
 
 The events for a small input, step by step, and what two different handlers do with them. Press *Play* or step with *Next*: