Make ox-koma-letter pass checkdoc.

Make ox-koma-letter pass checkdoc.

* ox-koma-letter.el: Reworded parts of the head of the file.
(org-koma-letter-prefer-special-headings,
org-koma-letter-subject-format, org-koma-letter-default-class,
org-koma-letter-special-tags-in-letter,
org-koma-letter-special-tags-after-closing,
org-koma-letter-special-tags-after-letter,
org-koma-letter-plug-into-ox, org-koma-letter-email,
org-koma-letter--get-tagged-contents,
org-koma-letter-special-contents): Changed docstring to pass checkdoc.
(org-koma-letter--get-value): Renamed `org-koma-letter--get-custom'.
Docstring changed to pass checkdoc.
(org-koma-letter--special-contents-as-macro): Renamed
`org-koma-letter--prepare-special-contents-as-macro'.  Docstring
changed to pass checkdoc.
(org-koma-letter--normalize-string): Renamed
org-koma-letter--remove-offending-new-lines.  Docstring changed to
pass checkdoc.
(org-koma-letter--determine-to-and-from): Renamed
org-koma-letter--determine-special-value.  Docstring changed to pass
checkdoc.

Some functions have been renamed to make the names slightly more
informative.

Signed-off-by: Alan Schmitt <alan.schmitt@polytechnique.org>