RFA/doc: Add agent expressions chapter

[Jim Blandy]

Andrew Cagney <ac131313@ges.redhat.com> writes:
> > We've got some nice documentation for all the tracepoint agent expressions
> > code, but we don't include it in the manual.  This patch adds that and fixes
> > a warning in the new chapter.  OK?
> >
> I wonder.  New chapter, new appendix (like the remote protocol), or
> part of gdbint?

The agent expression language is part of the interface between GDB and
a stub that implements tracepoints --- so it's a public interface, in
the same sense that the remote protocol is a public interface.  I
think it should go whereever the remote protocol docs go.