bug#58893: 29.0.50; Feedback on Eglot manual

[Felician Nemeth <felician.nemeth@gmail.com>]

All in all, I think the new eglot.texi is really informative.
Nevertheless, having reread Eglot's github page, I've found some
information that's missing from the manual.  Maybe it's worth including
some of them to the manual.

* Missing commands

I'm not sure whether it is necessary to document all the commands
defined by Eglot, but the manual mentions xref-find-definitions, so
probably it should call the reader's attention to
eglot-find-declaration, eglot-find-implementation,
eglot-find-typeDefinition, and xref-find-references as well.


* WorkspaceFolders

The LSP protocol supports multiple workspace folders.  Eglot relies on
project-external-roots to get the additional folders (besides
project-root).  This is not mentioned in the manual.  However, (info
"(emacs)Projects") doesn't talk about how the user should configure
these external roots.

Maybe a simple solution is to rework the relevant part from NEWS.md into
the eglot.texi. NEWS.md has this:

   Eglot now advertises project-root and project-external-roots as
   workspace-folders. (Configuring tags-table-list sets the external
   roots of a simple git project.)


* Snippet completion

Yasnippet should be enabled in the current buffer before Eglot connects
to the LSP server.


Thanks.