See http://issues.apache.org/SpamAssassin/show_bug.cgi?id=4771
There's now a wiki page that creates a prototype documentation page for a
rule:
http://wiki.apache.org/spamassassin/RulesList
Plug in a rule name and start documenting!
A little investigation reveals another path:
Go to the Tests page from the main web page:
http://spamassassin.apache.org/tests.html
Select the latest SA version to see its list of tests. For a given test
(AKA rule) click its Wiki link on the right. Either a descriptive page
already exists,