Decode text from any known FORMAT.
FORMAT is a symbol appearing in `format-alist' or a list of such symbols,
or nil, in which case this function tries to guess the format of the data by
matching against the regular expressions in `format-alist'. After a match is
found and the region decoded, the alist is searched again from the beginning
for another match.
Second arg LENGTH is the number of characters following point to operate on.
If optional third arg VISIT-FLAG is true, set `buffer-file-format'
to the reverted list of formats used, and call any mode functions defined
for those formats.
Return the new length of the decoded region.
For most purposes, consider using `format-decode-region' instead.