two small doc patches for doc index

[Ralf Wildenhues]

I've been looking a bit into the texinfo indexing stuff, see
<http://thread.gmane.org/gmane.comp.tex.texinfo.bugs/5088>
for an interesting thread that also highlights some problems with
autoconf's massive use of indices.

Anyway, let's start off with two small patches that fix a few very minor
glitches and make things a bit more consistent.

1) @cindex entries do not automatically get @code, so they may need
annotation.  Also, the m4_ and AS_ index could describe the naming
a bit better.

2) index variables which are used during configure or during
config.status.  Put them in vrindex, but merge that index into the
general concept index, in order to not increase the number of indices
further.

Cheers,
Ralf