docs: Fix up __sync_* documentation [PR117642]

docs: Fix up __sync_* documentation [PR117642]

The PR14311 commit which added support for __sync_* builtins documented that
there is a warning if a particular operation cannot be implemented.
But that commit nor anything later on implemented such warning, it was
always silent generation of the mentioned calls (which can in most cases
result in linker errors of course because those functions aren't implemented
anywhere, in libatomic or elsewhere in code shipped in gcc).

So, the following patch just adjust the documentation to match the
implementation.

2024-11-28  Jakub Jelinek  <jakub@redhat.com>

	PR target/117642
	* doc/extend.texi: Remove documentation of warning for unimplemented
	__sync_* operations, such warning has never been implemented.