[API suggestion] - Change return type for all HaveElement at XDocument and XElement

[ITaluone]

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Description

While implementing #1880 we discussed the return type for the new overloads for HaveElement(string | XName, OccurrenceConstraint). I realized that the "old" HaveElement overloads have a different return type (AndWhichConstraint<XDocumentAssertions, XElement>).

Now I would like to unify this and change the return type to AndWhichConstraint<XDocumentAssertions, IEnumerable<XElement>> as we discussed.

[ITaluone]

Hmm... after lookin deep into the meaning of AndWhichConstraint and the HaveElement implementation this is a completely unnecessary and more or less wrong "feature".