Clarify use of TMD files in user guide

docs/guide/python_interface.md

@@ -1,6 +1,14 @@
 Python interface
 ================
 
-Of the two ways of using Tax-Calculator described in this guide, one requires writing Python programs, the other involves using a command-line tool and requires no computer programming. The detailed information in this guide about policy parameters, input variables, output variables, and assumption parameters, will be useful to you either way you use Tax-Calculator.
+Of the two ways of using Tax-Calculator described in this guide, one
+requires writing Python programs, the other involves using a
+command-line tool and requires no computer programming. The detailed
+information in this guide about policy parameters, input variables,
+output variables, and assumption parameters, will be useful to you
+either way you use Tax-Calculator.
 
-If you want to learn more about how to write Python programs that use Tax-Calculator, follow the {doc}`../../recipes/index`, which contains a collection of tested recipes that employ both basic and advanced techniques.
+If you want to learn more about how to write Python programs that use
+Tax-Calculator, follow the {doc}`../../recipes/index`, which contains
+a collection of tested recipes that employ both basic and advanced
+techniques.

docs/usage/data.md

@@ -67,10 +67,16 @@ produces an input variables file (`tmd.csv`), a national weights file
 package beginning with the 3.6.0 release.  The `tmd.csv` file is
 available only to Tax-Calculator users who have purchased their own
 version of the 2015 IRS-SOI PUF.  For those users, those three files
-are avaiable from the tax-microdata repository.  These three tmd files
-can be used with the Tax-Calculator Python API (using the 
-`Records.tmd_constructor()` and `Policy.tmd_constructor()`static methods)
-or with the Tax-Calculator CLI tool, `tc`.
+are avaiable from the tax-microdata repository.  These three TMD files
+can be used with Tax-Calculator in two ways:
+
+  - with the **Python API** by instantiating a GrowFactors object that
+    uses TMD factors [gf = GrowFactors("path/to/tmd_growfactors.csv")]
+    and by using the `Records.tmd_constructor()` and
+    `Policy.tmd_constructor()`static methods
+
+  - with the **CLI tool**, `tc`, when the three TMD files are all in
+    the same folder
 
 ## Using other data with Tax-Calculator