Wire the --no-show-locs option to abidw

Wire the --no-show-locs option to abidw

Up until now, the --no-show-locs option only had effect when abidiff
was used with the --abidiff option.  It had no effect on the ABIXML
output of abidw.

This patch changes that to make it so that --no-show-locs avoids
emitting source location to the ABIXML output.

	* doc/manuals/abidw.rst: Update the documentation.
	* tests/test-annotate.cc: Now that --no-show-locs has an effect on
	the ABIXML output, let's not use it here, because it changes the
	output and we don't want that.
	* tools/abidw.cc (display_usage): Fix a typo in the help string.
	(load_corpus_and_write_abixml): Set the "show-locs" option to the
	write_context object that we use.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>