emacs: Improve doc-strings

- The first sentence should fit on the first line in full.  This is
  even the case when that causes the line to get a bit long.  If it
  gets very long, then it should be made shorter.

- Even even the second sentence would fit on the first line, if it
  just provides some details, then it shouldn't be done.

- Symbols are quoted like `so'.

- There is no clear rule on how to (not) quote non-atomic
  s-expressions, but quoting like '(this) is definitely weird.

- It is a good idea to remember that \" becomes " and to take
  that in mind when adjusting the automatic filling by hand.

- Use the imperative form.

- Arguments are written in all uppercase.