Update the manual to make DoF vs points clearer

docs/source/interpolation.rst

@@ -179,15 +179,16 @@ argument:
    :dedent:
    :lines: 72-73, 80
 
-By default the missing locations are set to zero:
+By default the missing degrees of freedom (DoFs, the global basis function
+coefficients which could not be set) are zero:
 
 .. literalinclude:: ../../tests/regression/test_interpolation_manual.py
    :language: python3
    :dedent:
    :lines: 85
 
-We can optionally specify a value to use for our missing locations. Here
-we set the missing locations to be ``nan`` (not a number) for easy identification:
+We can optionally specify a value to use for our missing DoFs. Here
+we set them to be ``nan`` ('not a number') for easy identification:
 
 .. literalinclude:: ../../tests/regression/test_interpolation_manual.py
    :language: python3
@@ -211,8 +212,8 @@ The ``default_missing_val`` keyword argument is then set whenever we call
    :lines: 103
 
 If we supply an output :py:class:`~.Function` and don't set
-``default_missing_val`` then any points outside the domain are left as they
-were prior to interpolation:
+``default_missing_val`` then any missing DoFs are left as they were prior to
+interpolation:
 
 .. literalinclude:: ../../tests/regression/test_interpolation_manual.py
    :language: python3