Document "no debug info debugging" improvements

Document "no debug info debugging" improvements

Here's the documentation bits for all the improvements done in
previous commits.

Note that the original "weak alias functions" paragraph ends up
disappearing, because this patch, which I'm considering kind of part
of this series, makes the alias case Just Work:
  https://sourceware.org/ml/gdb-patches/2017-07/msg00018.html

gdb/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* NEWS (Safer support for debugging with no debug info): New.

gdb/doc/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Variables) <Program Variables>: Document inspecting
	no-debug-info variables.
	(Symbols) <Examining the Symbol Table>: Document inspecting
	no-debug-info types.
	(Calling) <Calling functions with no debug info>: New subsection,
	documenting calling no-debug-info functions.
	(Non-debug DLL Symbols) <Working with Minimal Symbols>: Update.