obsstore: update create docstring to point to the coder friendly function

The `obsstore` class have a `create` method that create new obsolescence marker
from node. There is another function in the same module `createmarkers`. This
other function is higher level and automatically missing meta data (ultimately
calling the first one)

We add a new comment in the docstring of `obsstore.create` highlighting that
people writing new code probably want to use the top level one.

mercurial/obsolete.py

@@ -256,6 +256,9 b' class obsstore(object):'
         * ensuring it is hashable
         * check mandatory metadata
         * encode metadata
+
+        If you are a human writing code creating marker you want to use the
+        `createmarkers` function in this module instead.
         """
         if metadata is None:
             metadata = {}