The many rules for man pages and DocBook are a bit much to remember when
months can pass between working on the various types.

After not finding much to do automated checking, I slapped together a
Perl program called "igor" that does some of this.  At present it checks
all types of files for repeated words ("is is"), common spelling
mistakes collected from FreeBSD documents, FreeBSD obsolete features
(just "cvsup" so far), bad phrases ("the to"), and bad whitespace (blank
lines with whitespace or lines with trailing whitespace).
There's a separate style check that makes some subjective suggestions.

There are also particular tests for mdoc(7) and DocBook files.