Only use group in repeat as repeat instance name if it's the repeat's only child

In https://github.com/getodk/xforms-spec/issues/141, we decided to recommend that clients use the label of an inner group as a repeat instance label. We were not specific about what to do in case of other contents in the repeat.

https://github.com/getodk/xforms-spec/pull/309 makes a clarification of existing behavior in both Collect and Enketo: a group is only used this way if it's the repeat's first child.

In the original issue, we explicitly discussed the case where an inner group is followed by other items in the repeat but not in the group. We decided to still use the first group's label as the repeat instance label. This works ok in the Collect hierarchy view which is what this spec change was designed for.

It doesn't work as well in Enketo where multiple repeat instances are shown on a screen and can be individually collapsed.

Let's only use the inner group label this way if the inner group is the repeat's only child.

Read the context in xforms-spec issue 141 and the clarification in pull request 309 first. Locate the specification text describing repeat instance labels, then update it so the inner group label is used only when that group is the repeat's only child; done means the documented behavior matches this rule.

Written by the indexing model from the issue text.