bpo-37574: Mention helper functions for find_spec documentation (GH-14739)

Author: jdkandersson

Doc/library/importlib.rst

@@ -274,6 +274,8 @@ ABC hierarchy::
       parent package. If a spec cannot be found, ``None`` is returned.
       When passed in, ``target`` is a module object that the finder may
       use to make a more educated guess about what spec to return.
+      :func:`importlib.util.spec_from_loader` may be useful for implementing
+      concrete ``MetaPathFinders``.
 
       .. versionadded:: 3.4
 
@@ -323,7 +325,8 @@ ABC hierarchy::
       within the :term:`path entry` to which it is assigned.  If a spec
       cannot be found, ``None`` is returned.  When passed in, ``target``
       is a module object that the finder may use to make a more educated
-      guess about what spec to return.
+      guess about what spec to return. :func:`importlib.util.spec_from_loader`
+      may be useful for implementing concrete ``PathEntryFinders``.
 
       .. versionadded:: 3.4