Clarification of ical integration

[plantarum]

I just got mu4e-icalendar setup, and it looks great. However, the documentation isn't quite complete, at least as far as I can see. For those of us that don't use Gnus, it's not clear that you need to set mu4e-view-use-gnus to make it work. i.e., :

;; Use gnus article view
(setq mu4e-view-use-gnus t)

(require 'mu4e-icalendar)
(mu4e-icalendar-setup)
;; Optional
;; (setq mu4e-icalendar-trash-after-reply t)

Perhaps that could be added to the preamble of mu4e-icalendar.el, or added to the actual manual?

Originally posted by @peterwvj in #1403 (comment)

[djcb]

Indeed, a few lines in the manual would be good; the 'Other tools' appendix would be a good place. PRs are welcome :)

[plantarum]

Done! I haven't written texinfo in a while. I tried to satisfy all the warnings that makeinfo generated.