Rename a few sections.

Author: ZeroIntensity

peps/pep-9999.rst

@@ -155,8 +155,8 @@ The ``yield_expr`` and ``simple_stmt`` rules need to be updated for the new
         | &('yield' | 'async') yield_stmt
 
 
-``yield from`` semantics
-------------------------
+``yield from`` behavior in asynchronous generators
+--------------------------------------------------
 
 This PEP retains all existing ``yield from`` semantics; the only detail is
 that asynchronous generators may now use it.
@@ -181,7 +181,7 @@ In the above code, ``main`` will yield ``1``, ``2``, ``3``, ``4``.
 All subgenerator delegation semantics are retained.
 
 
-``async yield from`` base semantics
+``async yield from`` as a statement
 -----------------------------------
 
 ``async yield from`` is equivalent to ``yield from``, with the exception that: