diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -1114,6 +1114,9 @@ def diff(ui, repo, *pats, **opts):
     revisions are specified, the working directory files are compared
     to its parent.
 
+    Alternatively you can specify -c/--change with a revision to use
+    as a baseline.
+
     Without the -a/--text option, diff will avoid generating diffs of
     files it detects as binary. With -a, diff will generate a diff
     anyway, probably with undesirable results.
@@ -3552,7 +3555,7 @@ table = {
          [('r', 'rev', [], _('revision')),
           ('c', 'change', '', _('change made by revision'))
          ] + diffopts + diffopts2 + walkopts,
-         _('[OPTION]... [-r REV1 [-r REV2]] [FILE]...')),
+         _('[OPTION]... ([-c REV] | [-r REV1 [-r REV2]]) [FILE]...')),
     "^export":
         (export,
          [('o', 'output', '', _('print output to file with formatted name')),
diff --git a/tests/test-help.out b/tests/test-help.out
--- a/tests/test-help.out
+++ b/tests/test-help.out
@@ -366,7 +366,7 @@ verify the integrity of the repository
     and indices.
 
 use "hg -v help verify" to show global options
-hg diff [OPTION]... [-r REV1 [-r REV2]] [FILE]...
+hg diff [OPTION]... ([-c REV] | [-r REV1 [-r REV2]]) [FILE]...
 
 diff repository (or selected files)
 
@@ -383,6 +383,9 @@ diff repository (or selected files)
     compared to the working directory, and, when no revisions are specified,
     the working directory files are compared to its parent.
 
+    Alternatively you can specify -c/--change with a revision to use as a
+    baseline.
+
     Without the -a/--text option, diff will avoid generating diffs of files it
     detects as binary. With -a, diff will generate a diff anyway, probably
     with undesirable results.