Dear PG,
you're welcome to make your contributions via Mantis Bug Tracker including documentation improvements. If you wish to improve a particular piece of OCCT documentation you may do so by registering a corresponding bug and by adding your documentation and whatever else is necessary to the Bug Description Field. As soon as this is done your contribution will be integrated to the current documentation (whether it's a User's Guide or something else) and will be available with the next OCCT release.
Regards
Commenter-1