Document linespec/explicit locations & completion improvements (manual + NEWS)

gdb/ChangeLog:
2017-11-24  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention linespecs and explicit locations, and completion
	improvements.

gdb/doc/ChangeLog:
2017-11-24  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Completion): Update need-quoting examples.  Remove
	false claim that GDB inserts quoting automatically.
	(Symbols): Add anchor.