Return annotations for writing region as FORMAT.
FORMAT is a symbol naming one of the formats defined in `format-alist'.
It must be a single symbol, not a list like `buffer-file-format'.
FROM and TO delimit the region to be operated on in the current buffer.
ORIG-BUF is the original buffer that the data came from.
FORMAT-COUNT is an integer specifying how many times this function has
been called in the process of decoding ORIG-BUF.
This function works like a function in `write-region-annotate-functions':
it either returns a list of annotations, or returns with a different buffer
current, which contains the modified text to write. In the latter case,
this function's value is nil.
For most purposes, consider using `format-encode-region' instead.