(Compilation): Move and split kill-compilation para.

(Compilation): Move and split kill-compilation para.
Add para about multiple compilers.
(Compilation Mode): Commands also available in grep mode and others.
Mention C-u C-x ` more tutorially.  Clarify C-x `.
(Compilation Shell): Clarify.  Put Bash example first.
(Grep Searching): Minor cleanups; add @w.
(Debuggers): Minor cleanups.
(Starting GUD): Make GDB xgraphical mode issue clearer.
(Debugger Operation): Lots of clarifications including
GDB tooltip side-effect issue.
(Commands of GUD): Clarify.
(GUD Customization): Add bashdb-mode-hook.
(GDB Graphical Interface): Rewrite for clarity.
(GDB User Interface Layout): Rewrite for clarity.
(Stack Buffer, Watch Expressions): Likewise.
(Other GDB User Interface Buffers): Cleanups.
(Lisp Libraries, External Lisp): Cleanup.