Fix docstrings of `edit-server-log-buffer-name'.

It's not a template, since it's used with `get-buffer-create'.
Style fixes.

Two misindented line, one use of ";" that should be ";;", a couple of
`if's that are better written as `when's, missing space, typo, avoid
calling `get-buffer' with "".
Fix header searching.

Remove two CR characters that are not needed (since the regexp string
already has a "\r"), and also remove the strange "?" from the regexp (my
best guess is that this is a mistake due to some earlier attemt that
used the popular "\r?" pattern).
Fix a bug in `edit-server-choose-major-mode'.

Looks like commit a4e1dbe1 mistakenly dropped the fallback to
`edit-server-default-major-mode'.  Bring it back.  No need for the test
now, since `edit-server-default-major-mode' can't be nil (according to
its declaration).
Update docs with latest bug fix.