HPCC-11031 Document MAXLENGTH Options

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>
"If flag is flag" ?

I'm not really sure what the proper behaviour of 'FILEPOSITION' is.
This paragraph makes it sound like, that if present, it will _not_ treat the trailing integer as anything special (as it does by default). 
@ghalliday - can you comment?