Previously, the code which parsed comments in key files would append a
line break to the comment where there was none before; this was part of
the code for handling re-inserting line breaks into multi-line comments
after removing the ‘#’ prefix. Now, we don’t add a terminal line break.

This was slightly icky to implement because parse_value_as_comment() is
called once for each line of a multi-line comment.

This expands the existing test case to cover a single line comment, and
also fixes the documentation to correctly state that the leading ‘#’
*is* removed and mention the new line break behaviour.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

#107