Improve load command's help text

This fairly obvious patch adds usage text to the load command's help text.

Originally it did not have usage and mentioned things like FILE and OFFSET
without explaining how those should be passed in the command.

gdb/ChangeLog:

2017-02-13  Luis Machado  <lgustavo@codesourcery.com>

	* symfile (_initialize_symfile): Add usage text to the load command's
	help text.

gdb/doc/ChangeLog:

2017-02-13  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.texinfo (Target Commands): Document the optional offset
	argument for the load command.