ox-koma-letter: Cleanup some docstrings
* contrib/lisp/ox-koma-letter.el (org-koma-letter-author,
org-koma-letter-email, org-koma-letter-place,
org-koma-letter-prefer-special-headings, org-koma-letter-signature,
org-koma-letter-subject-format, org-koma-letter-use-backaddress,
org-koma-letter-use-phone, org-koma-letter-use-email,
org-koma-letter-use-place,
org-koma-letter-headline-is-opening-maybe,
org-koma-letter--get-tagged-contents,
org-koma-letter--format-string-as-macro,
org-koma-letter--normalize-string): Cleanup docstrings.