perf: document `perfdirstatewrite`

contrib/perf.py

@@ -1185,6 +1185,8 b' def perfdirfoldmap(ui, repo, **opts):'
 
 @command(b'perfdirstatewrite', formatteropts)
 def perfdirstatewrite(ui, repo, **opts):
+    """benchmap the time it take to write a dirstate on disk
+    """
     opts = _byteskwargs(opts)
     timer, fm = gettimer(ui, opts)
     ds = repo.dirstate

tests/test-contrib-perf.t

@@ -113,7 +113,7 b' perfstatus'
    perfdirstatefoldmap
                  benchmap a 'dirstate._map.filefoldmap.get()' request
    perfdirstatewrite
-                 (no help text available)
+                 benchmap the time it take to write a dirstate on disk
    perfdiscovery
                  benchmark discovery between local repo and the peer at given
                  path