More updates to Building chapter of Emacs manual.

More updates to Building chapter of Emacs manual.

* doc/emacs/building.texi (Compilation): Say what the -k flag to make does.
Move subprocess discussion to Compilation Shell.
(Compilation Mode): Add xref for grep, occur, and mouse
references.  Define "locus".
(Grep Searching): Use @command.
(Debuggers, Commands of GUD, GDB Graphical Interface): Clarify
intro.
(Starting GUD): Clarify how arguments are specified.
(Debugger Operation): Index entry for "GUD interaction buffer",
and move basic description here from Commands of GUD node.
(GDB User Interface Layout): Copyedits.
(Source Buffers): Remove gdb-find-source-frame, which is not in
gdb-mi.el.
(Other GDB Buffers): Remove gdb-use-separate-io-buffer and
toggle-gdb-all-registers, which are not in gdb-mi.el.  Don't
re-document GUD interaction buffers.