Add "usage" text to all TUI command help

Add "usage" text to all TUI command help

This adds "usage" text to the help for all all the TUI commands.  In
some cases the usage is borderline, but I tend to think being complete
is preferable.

2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-win.c (_initialize_tui_win): Add usage text.
	* tui/tui-stack.c (_initialize_tui_stack): Add usage text.
	* tui/tui-regs.c (_initialize_tui_regs): Add usage text.
	* tui/tui.c (_initialize_tui): Add usage text.

Change-Id: I727f7a7cfc03efa248ef98f30a18be393819e30b