Docs: Update documentation of Texinfo versions for building manuals.

There has been recent discussion on updating the minimum required
version of Texinfo from the current version 4.7.  This patch does not
do that, but it suggests that people use a more recent version to get
better output.  It also removes some other references to Texinfo 4.7
and fixes some related bit-rot in the installation manual.  (Nobody
really wants to print the GCC manual any more, and the GCC web site
is a better place to get prebuilt manuals than the FSF store.)

gcc/ChangeLog:
	* doc/install.texi (Prerequisites): Suggest using newer versions
	of Texinfo.
	(Final install): Clean up and modernize discussion of how to
	build or obtain the GCC manuals.
	* doc/install.texi2html: Update comment to point to the PR instead
	of "makeinfo 4.7 brokenness" (it's not specific to that version).