binman: Tweak collect_contents_to_file() and docs

Update the return value of this function, fix the 'create' typo and
update the documentation for clarity.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Suggested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>