Increase `emacs-lisp-docstring-fill-column` to 72

Monitors are wider now than when these defaults were first set, and it
is useful to take better advantage of that, to fit text on fewer lines.
Yet, it has repeatedly been shown that overly long lines reduce
readability:
   "A reasonable guideline would be 55 to 75 characters per line."[1]

We also don't want to disfavor narrow displays, like mobile phones; a
more promising direction here might be to automatically word wrap
docstrings and make their maximum width customizable.  That might
require a new docstring format, however.

Bumping it by 7 characters, from 65 to 72, seems a reasonable compromise
for now.  Consideration was given to increasing it to 70 or 75, but 72
happens to be a commonly recommended maximum line width elsewhere (see
Fortran 66, Python docstrings, commit message recommendations, etc.),
and we might as well do the same.

This change was discussed in:
https://lists.gnu.org/r/emacs-devel/2022-07/msg00217.html

[1] "Optimal Line Length in Reading — A Literature Review", Nanavati and
    Bias, Visible Language, Vol. 39 No. 2 (2005).
    https://journals.uc.edu/index.php/vl/article/view/5765

* lisp/emacs-lisp/lisp-mode.el (emacs-lisp-docstring-fill-column):
* .dir-locals.el (fill-column, emacs-lisp-docstring-fill-column):
Bump default to 72.

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