Improve "backtrace" help text

This improves help text in stack.c in two ways.  First, it removes
trailing newlines from various help strings.  I think these are never
needed.  Second, it adds a "Usage" line to the "backtrace" text, as
suggested by Pedro.

gdb/ChangeLog
2018-03-26  Tom Tromey  <tom@tromey.com>

	* stack.c (_initialize_stack): Remove trailing newlines from help
	text.  Add "Usage" line to "backtrace" help.