log: document the characters ---graph uses to draw

The meaning of : vs | was undocumented and non-obvious.

mercurial/commands.py

@@ -3334,6 +3334,8 b' def log(ui, repo, *pats, **opts):'
     'o' is a changeset, '@' is a working directory parent, 'x' is obsolete,
     and '+' represents a fork where the changeset from the lines below is a
     parent of the 'o' merge on the same line.
+    Paths in the DAG are represented with '|', '/' and so forth. ':' in place
+    of a '|' indicates one or more revisions in a path are omitted.
 
     .. note::