More minor fixes to doc/misc/eglot.texi

More minor fixes to doc/misc/eglot.texi

Most, if not all of these, were previously discussed with Eli.

* doc/misc/eglot.texi (Setting Up LSP Server): Fix repetition of
"This variable".
(Setting Up LSP Server): Explain that single a running instance
supports multiple major modes.
(Starting Eglot, Eglot and Buffers): Correctly describe mode-line indication.
(Eglot Features): Suggest that company-mode is just one of the
possible packages.  Explain that Eglot arranges for the completion
package to "instantiate" snippets.  Could have used "expand".
Mention benefits of having the popular markdown-mode available.
(Eglot Commands): Explain how eglot-reconnect and
eglot-clear-status are useful.
(Eglot Variables): Clarify when eglot-autoreconnect is useful.
Clarify how eglot-sync-connect and eglot-connect-timeout relate to
each other.  Clarify semantics of
eglot-confirm-server-initiated-edits.