perf: document `perfdirstatefoldmap`

contrib/perf.py

@@ -1142,6 +1142,10 b' def perfdirstatedirs(ui, repo, **opts):'
 
 @command(b'perfdirstatefoldmap', formatteropts)
 def perfdirstatefoldmap(ui, repo, **opts):
+    """benchmap a `dirstate._map.filefoldmap.get()` request
+
+    The dirstate filefoldmap cache is dropped between every request.
+    """
     opts = _byteskwargs(opts)
     timer, fm = gettimer(ui, opts)
     dirstate = repo.dirstate

tests/test-contrib-perf.t

@@ -111,7 +111,7 b' perfstatus'
    perfdirstatedirs
                  benchmap a 'dirstate.hasdir' call from an empty 'dirs' cache
    perfdirstatefoldmap
-                 (no help text available)
+                 benchmap a 'dirstate._map.filefoldmap.get()' request
    perfdirstatewrite
                  (no help text available)
    perfdiscovery