document about manuals and .texi format

[SaranjeetKaur]

The manuals do not use the same infrastructure as the help files. Specifically, as a GNU project, R uses GNU Texinfo to create the R manuals (https://www.gnu.org/software/texinfo/), so the source of the manuals are .texi files, e.g. https://github.com/r-devel/r-svn/blob/master/doc/manual/R-exts.texi, with different markup from .Rd files. Since .texi is not an R-specific format there is no need to document it fully here, but we should document that manuals use this format. You may need to create a new issue to add this in later in the chapter.

Also people may suggest changes without providing a formal patch.

I'm not sure that proposals for completely new documentation would get very far - that would certainly be unusual, so we shouldn't promote that as a typical way to contribute.

Originally posted by @hturner in https://github.com/r-devel/rdevguide/pull/144#discussion_r1203826587