xantispam: added new rule &-IM/GRLogFullHistory

	   This changes commit
	   8589ddccf4e1a63f76d93d4c4cbf60fac8e5c90c in that whether
	   the whole file is loaded or not now depends on the rule
	   being in effect rather than on LogShowHistoryLines being
	   zero.

	   The rule does not apply to the local chat log:  "chat.txt"
	   is always loaded as if xantispam didn't exist.

	   This avoids possible performance problems of the internal
	   text editor with large files, particularly with the local
	   chat log, and as well with other communications since the
	   user can decide which to load in full and which not.
added documentation for xantispam

The documentation is not finished and work in progress.