Clarify and simplify language around object methods in sections.yml.

See issue mustache#46 for related discussion:

  mustache#46

specs/sections.yml

@@ -15,12 +15,9 @@ overview: |
     to a method with the given name.
     3) If the context is a hash, the data is the value associated with the
     name.
-    4) If the context is an object and the method with the given name has an
-    arity of 1, the method SHOULD be called with a String containing the
-    unprocessed contents of the sections; the data is the value returned.
-    5) Otherwise, the data is the value returned by calling the method with
-    the given name.
-    6) If any name parts were retained in step 1, each should be resolved
+    4) If the context is an object, the data is a lambda equal to the method
+    associated with the name.
+    5) If any name parts were retained in step 1, each should be resolved
     against a context stack containing only the result from the former
     resolution.  If any part fails resolution, the result should be considered
     falsey, and should interpolate as the empty string.