emacs: inline notmuch-documentation-first-line

Inline a simplified version of `notmuch-documentation-first-line'
into its only caller.  The new code snippet differs from the
removed function in that it returns nil instead of the empty string
for symbols that have no function documentation.  That value is
ultimately used as an argument to `concat', which treats nil like
the empty string.  So we can do the logical thing without changing
the behavior.