Update usage text for add-symbol-file, symbol-file, and load

This updates the usage text for the add-symbol-file, symbol-file, and
load commands.

gdb/ChangeLog
2017-11-29  Tom Tromey  <tom@tromey.com>

	* symfile.c (_initialize_symfile): Update usage text for
	add-symbol-file, symbol-file, load.