Document (or deprecate?) the "instance_xmlns" setting

[lindsay-stevens]

Software and hardware versions

pyxform v1.1.0, Python 3.8

Problem description

It's possible to set a custom namespace on the form data instance element, as follows:

settings sheet

instance_xmlns
1234

survey sheet

type	name	label
text	q1	hello

results snippet

        <model odk:xforms-version="1.0.0">
            <instance>
                <test_name xmlns="1234" id="test_id">
                    <q1/>
                    <meta>
                        <instanceID/>
                    </meta>
                </test_name>
            </instance>

Main code here, test code here, added by this commit in 2012 "to make commcare happy".

This feature doesn't appear in the XLSForms docs or the XForms spec, but it's been there for 10+ years and has a test. Should it be documented, or deprecated?