Suggest when testing Apps within libraries is helpful

E.g., join apps and other complex patterns.

Thanks, @Andrew-S-Rosen, for noting where he uses Parsl inside the
library.

docs/userguide/apps.rst

@@ -196,6 +196,8 @@ Learn more about the types of data allowed in `the data section <data.html>`_.
 
     Any changes to mutable input arguments will be ignored.
 
+.. _special-kwargs:
+
 Special Keyword Arguments
 +++++++++++++++++++++++++
 

docs/userguide/modularizing.rst

@@ -69,6 +69,9 @@ Key points to note:
   Keeping Parsl out of the function definitions simplifies testing
   because you will not need to run Parsl when testing the code.
 
+- *Advanced*: Consider including Parsl decorators in the library if using complex workflow patterns,
+  :ref:`join apps <label-joinapp>` or such as functions which take :ref:`special arguments <special-kwargs>`.
+
 .. literalinclude:: examples/library/app.py
    :caption: library/app.py