Re: Overlay arrow in *compilation* and *grep* buffers

[Nick Roberts]

Richard Stallman writes:
 > Please install your patch, with Eli's doc string.
 > 
 > Please update etc/NEWS and the Emacs Manual too.

I've comitted these changes. The manual refers to "error messages" but these
can be "warning messages" too. As with compile.el, I've just called them
messages, I guess the correct term would be "diagnostic messages". All
the functions are next-error-* and changing them to next-diagnostic-*
wouldn't be practical. However, I do think it does make the manual unclear.

While debugging these lisp changes with Edebug, I noticed that `g' (top-level)
didn't clear Edebug's arrow. I don't know if this was due to the compilation
buffer having a local value but it doesn't happen on other functions when the
compilation buffer isn't present. It might be cleaner to give compile its
own arrow like GUD and next-error (the source buffer based arrow).

Nick