perf: document `perfdirfoldmap`

contrib/perf.py

@@ -1163,6 +1163,10 b' def perfdirstatefoldmap(ui, repo, **opts'
 
 @command(b'perfdirfoldmap', formatteropts)
 def perfdirfoldmap(ui, repo, **opts):
+    """benchmap a `dirstate._map.dirfoldmap.get()` request
+
+    The dirstate dirfoldmap cache is dropped between every request.
+    """
     opts = _byteskwargs(opts)
     timer, fm = gettimer(ui, opts)
     dirstate = repo.dirstate

tests/test-contrib-perf.t

@@ -105,7 +105,7 b' perfstatus'
    perfctxfiles  (no help text available)
    perfdiffwd    Profile diff of working directory changes
    perfdirfoldmap
-                 (no help text available)
+                 benchmap a 'dirstate._map.dirfoldmap.get()' request
    perfdirs      (no help text available)
    perfdirstate  benchmap the time necessary to load a dirstate from scratch
    perfdirstatedirs