libgdiagnostics: doc fixes

libgdiagnostics: doc fixes

gcc/ChangeLog:
	* doc/libgdiagnostics/topics/compatibility.rst
	(_LIBGDIAGNOSTICS_ABI_2): Add missing anchor.
	* doc/libgdiagnostics/topics/diagnostic-manager.rst
	(diagnostic_manager_add_sink_from_spec): Add links to GCC's
	documentation of "-fdiagnostics-add-output=".  Fix parameter
	markup.
	(diagnostic_manager_set_analysis_target): Fix parameter markup.
	Add link to SARIF spec.
	* doc/libgdiagnostics/topics/logical-locations.rst: Markup fix.
	* doc/libgdiagnostics/tutorial/02-physical-locations.rst: Clarify
	wording of what "the source file" means, and that a range can't
	have multiple files.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>