org-agenda.el: Allow `org-agenda-finalize-hook' to modify the buffer

org-agenda.el: Allow `org-agenda-finalize-hook' to modify the buffer

* org-agenda.el (org-agenda-before-write-hook)
(org-agenda-add-entry-text-maxlines): Enhance phrasing.
(org-agenda-finalize-hook, org-agenda-mode-hook): Tell that
the buffer is writable when the hook is called.
(org-agenda-finalize): Allow org-agenda-finalize-hook to
modify the buffer.

Thanks to Moritz Ulrich and to Aaron Ecay for triggering this fix.