Explicit locations: documentation updates

Explicit locations: documentation updates

This patch adds documentation for explicit locations to both the
User Manual and gdb's online help system.

gdb/ChangeLog:

	* NEWS: Mention explicit locations.
	* breakpoint.c [LOCATION_HELP_STRING]: New macro.
	[BREAK_ARGS_HELP]: Use LOCATION_HELP_STRING.
	(_initialize_breakpoint): Update documentation for
	"clear", "break", "trace", "strace", "ftrace", and "dprintf".

gdb/doc/ChangeLog:

	* gdb.texinfo (Thread-Specific Breakpoints, Printing Source Lines):
	Use "location(s)"instead of "linespec(s)".
	(Specifying a Location): Rewrite.
	Add subsections describing linespec, address, and explicit locations.
	Add node/menu for each subsection.
	(Source and Machine Code, C Preprocessor Macros)
	(Create and Delete Trace points)
	(Extensions for Ada Tasks): Use "location(s)" instead of "linespec(s)".
	(Continuing at a Different Address): Remove "linespec" examples.
	Add reference to "Specify a Location"
	(The -break-insert Command): Rewrite.  Add anchor.
	Add reference to appropriate manual section discussing locations.
	(The -dprintf-insert Command): Refer to -break-insert for
	specification of 'location'.

gdb/testsuite/ChangeLog:

	* gdb.base/help.exp: Update help_breakpoint_text.