dirstate: document the dirstatetuple content

We are about to modify this, so let us document the existing code.

Differential Revision: https://phab.mercurial-scm.org/D10935

mercurial/pure/parsers.py

@@ -35,6 +35,13 b' stringio = pycompat.bytesio'
 # Some code below makes tuples directly because it's more convenient. However,
 # code outside this module should always use dirstatetuple.
 def dirstatetuple(*x):
+    """the four items are:
+    - state (one of 'n', 'a', 'r', 'm')
+    - mode,
+    - size,
+    - mtime,
+    """
+
     # x is a tuple
     return x