Remove the binding of 'C-M-i' to 'ispell-complete-word' in Text mode.
Define a new 'ispell-completion-at-point' function and add that to
'completion-at-point-functions' in Text mode, such that
'completion-at-point' provides the same word completions as
'ispell-complete-word' does OOTB.

* lisp/textmodes/ispell.el (ispell-completion-at-point): New function.
* lisp/textmodes/text-mode.el (text-mode): Add it to 'c-a-p-functions'.
(text-mode-map): Remove 'C-M-i' binding.
(text-mode-meta-tab-ispell-complete-word): New user option.
* etc/NEWS: Announce it.
* doc/emacs/fixit.texi (Spelling)
* doc/emacs/text.texi (Text Mode)
* doc/lispref/modes.texi (Basic Major Modes)
(Example Major Modes): Update.
* lisp/mail/sendmail.el (mail-abbrevs-loaded)
* lisp/nxml/nxml-mode.el (nxml-mode-map): Remove superfluous binding.

* lisp/nxml/rng-util.el (rng-substq): Simplify.
(cl-lib): Require.

* lisp/net/shr.el (shr-h4, shr-h5, shr-h6):
* lisp/cedet/semantic/decorate/include.el
(semantic-decoration-on-includes):
* lisp/progmodes/make-mode.el (makefile-shell):
* lisp/time.el (display-time-date-and-time):
* lisp/nxml/nxml-mode.el (nxml-text, nxml-delimiter)
(nxml-element-colon): Add face definitions (bug#64655).

* lisp/nxml/rng-nxml.el (rng-complete-tag): Add the (ignored)
argument to the :company-kind function.  Fixes the "Wrong number
of arguments" error reported at
https://github.com/company-mode/company-mode/issues/1386.

* lisp/nxml/xmltok.el (xmltok-scan-attributes): Limit the search
to 10000 characters, to avoid slowdowns due to the quadratic
complexity of the regexp.  Suggested by Stefan Monnier.