Document updates

[Ikumi Keita]

Hi all,

I committed update on documents. Although their size is rather large,
most of them are just fixes of markups so their contents don't change so
much.

I'd like to enumerate a few notable exceptions here:

1. The issue briefly discussed before[1] exists in texinfo document as
well, so there is currently wrong descrption about the simultaneity
between master- and region-process.
[1] https://lists.gnu.org/r/auctex-devel/2020-11/threads.html

2. The document states that AUCTeX treats file which matches
`TeX-header-end' as master file, but it seems no such feature is
implmented in the current source.

3. The document states that Texinfo mode can run texindex through
"Index" entry in `TeX-command-list', but "Index" actually runs
makeindex, not texindex. No entry in `TeX-command-list' runs texindex.
Should we add a new entry in `TeX-command-list' to run texindex?
(It seems to me more useful for users to enable running texi2pdf than
just texindex, so I suppose that it should be such entry rather than
texindex entry that is to be added.)

Comments and discussions are welcome.

Best regards,
Ikumi Keita