Proposal: support <hint> element as the child element of <group>

Currently, in XLSForm, we can fill in the hint column of the group and repeat, however, the hint will not be converted to xform.

Start by reading the specification's treatment of group and repeat elements, then trace how the XLSForm hint column is represented in the corresponding XForm structure. Confirm the expected child-element form and verify that hints on groups and repeats are converted consistently; the issue names no files or tests, so the relevant specification entry points must first be located.

Written by the indexing model from the issue text.