help: document meaning of '%' in graphlog output

I added the feature in 14d0e89520a2 (graphlog: use '%' for other
context in merge conflict, 2020-01-28), but I didn't think to look for
documentation until today (because an internal user asked about it).

Differential Revision: https://phab.mercurial-scm.org/D8649

mercurial/commands.py

@@ -4596,7 +4596,8 b' def log(ui, repo, *pats, **opts):'
 
     With --graph the revisions are shown as an ASCII art DAG with the most
     recent changeset at the top.
-    'o' is a changeset, '@' is a working directory parent, '_' closes a branch,
+    'o' is a changeset, '@' is a working directory parent, '%' is a changeset
+    involved in an unresolved merge conflict, '_' closes a branch,
     'x' is obsolete, '*' is unstable, and '+' represents a fork where the
     changeset from the lines below is a parent of the 'o' merge on the same
     line.