Document pulldata and provide examples

Pulldata is now officially supported: https://opendatakit.github.io/xforms-spec/#fn:pulldata

The documentation should state that it is particularly useful for CSVs and in fact that Collect only supports it for CSVs currently (until https://github.com/opendatakit/javarosa/issues/427 is addressed). The spec describes it in terms of elements but it's really only useful to users who use CSVs so this user-facing documentation should describe it in terms of columns (e.g. `pulldata(, , , )`).

Some useful information can be brought in from https://github.com/opendatakit/docs/issues/73.

Start by locating the user-facing documentation for pulldata in this Sphinx documentation repository, then review the XForms specification link and the related docs issue 73. Add an explanation framed around CSV columns, include examples, and document that Collect currently supports pulldata only for CSVs until javarosa issue 427 is addressed.

Written by the indexing model from the issue text.