Documentation #5182: userguide: better document rule keywords
file.name sticky buffer is not documented
The file.name sticky buffer is not documented. It should be documented in /rules/file-keywords.html#filename as the code in detect-filename.c points, but only the keyword filename is documented.
Moreover, there should be references to this sticky buffer in all related protocols documentation where is used, in order to be easier a user to find it. The related protocols are: