doc: clarify adding ctx argument to check command in schema.py

On IRC user was unsure what file was used for the 1.51 - 1.60 upgrade
section:

  Enhancement to check command for Permissions

added file name desription and example. Also marked it as optional.
Clarified that if or when it becomes required there will be a new
required upgrade direction.

Author: rouilj

doc/upgrading.txt

@@ -2885,12 +2885,13 @@ If you haven't modified you query.item.html template, simply copy the
 query.item.html template from one of the above default templates to
 your tracker's html directory.
 
-Enhancement to check command for Permissions
---------------------------------------------
-
+Enhancement to check command for Permissions (optional)
+-------------------------------------------------------
 A new form of check function is permitted in permission definitions.
-The three argument form is still supported and will work the same
-as it always has (although it may be depricated in the future).
+An example check function is ``own_record(db, userid, itemid)`` in the
+file schema.py. The three argument form is still supported and will
+work the same as it always has (although it may be depricated in the
+future).
 
 If the check function is defined as::
 
@@ -2936,6 +2937,9 @@ parameter. This is expected to be the preferred form in the future.
 You do not need to use the ``ctx`` parameter in the function if you do
 not need it.
 
+If the new four argument form is required in the future, there will be
+required (not optional) directions on upgrading your schema.
+
 Changes to property permissions
 -------------------------------