docs: more details about make VPATH rewriting woes

docs: more details about make VPATH rewriting woes

* doc/autoconf.texi (Automatic Rule Rewriting): Solaris make
VPATH rewriting applies to any whitespace-separated word in a
rule, so it might apply also to shell variables, functions
and keywords (and automake has already tripped on this once);
document this, with an example.  Since we are at it, do some
minor reformatting of existing text.