tests: document what the other currently enabled pylint check ensures

tests/test-check-pylint.t

@@ -1,22 +1,23 b''
 #require test-repo pylint
 
 Run pylint for known rules we care about.
 -----------------------------------------
 
 There should be no recorded failures; fix the codebase before introducing a
 new check.
 
 Current checks:
 - W0102: no mutable default argument
+- C0321: only one statement on a single line
 
   $ touch $TESTTMP/fakerc
   $ pylint --rcfile=$TESTTMP/fakerc --disable=all \
   >   --enable=W0102,C0321 \
   >   --reports=no \
   >   --ignore=thirdparty \
   >   mercurial hgdemandimport hgext hgext3rd | sed 's/\r$//'
   Using config file *fakerc (glob) (?)
    (?)
   ------------------------------------* (glob) (?)
   Your code has been rated at 10.00/10* (glob) (?)
    (?)