Documentation -- part two

Pushing this change to continue work elsewhere.
Simplify `sx-method-call'

We aren't using `stack-filter' anymore (in fact, we haven't since
filters became functional).  Somehow it survived refactoring; this
commit fixes that oversight.
Documentation -- part three

Source code is documented to the best of my knowledge and understanding.
Update user manual

Removed developer reference since this should be embedded in docstrings
for the most part.