bug#50801: 28.0.50; Emacs manual about file variables doesn't mention that major modes shouldn't end in "-mode"

["Daniel Martín via Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

I'm reviewing the manuals for the upcoming Emacs 28 and I have a couple
of minor suggestions for node "49.2.4.1 Specifying File Variables":

The node doesn't explictly mention the crucial information that in
'mode: MODENAME' 'MODENAME' must not end in "-mode".  It's true that
there's an example down there that says "mode: Lisp", so users may infer
that you don't need to spell the major mode with "-mode", but I thought
it may be better to be explicit about in the manual.

Another suggestion I have is that the manual doesn't mention that
something like "-*- C++ -*-" (that is, omitting the "mode" keyword) is
also supported.  Many free software projects have traditionally been
using "-* C++ -*" in their C++ header files to make Emacs distinguish
them from plain C files.  See also src/epaths.in in the Emacs
repository, among others.  I see this used by Jim Blandy since 1992 at
least, but I don't really know if a) it's really a stable way to declare
a file-local mode, and we may lack the historical info to answer that,
and b) it's worth mentioning it in the manual.

Thanks.