log: describe graph symbols in the help

I wondered what 'x' meant and didn't find it described anywhere.

mercurial/commands.py

@@ -3905,6 +3905,12 b' def log(ui, repo, *pats, **opts):'
     each commit. When the -v/--verbose switch is used, the list of
     changed files and full commit message are shown.
 
+    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+    'o' is a changeset, '@' is a working directory parent, 'x' is obsolete,
+    and '+' represents a fork where the changeset from the lines below is a
+    parent of the 'o' merge on the same same line.
+
     .. note::
 
        log -p/--patch may generate unexpected diff output for merge