#633 @source in att.editLike is overly constrained

[Martin Holmes]

The remarks for @source in att.editLike says:

<p>Each pointer on the <att>source</att> (if present)
    corresponding to a witness or witness group should indicate a
    <gi>witness</gi> or <gi>msDesc</gi> element documenting the source
    concerned.</p>

I don't see why there should be a requirement to point to a <witness> or <msDesc> element; why could one not point to an external source, or a <bibl> element?

Note: this partly relates to the discussion on the previously-implemented FR for @source in att.source/att.responsibility:

https://sourceforge.net/p/tei/feature-requests/465/

[James Cummings]

Assigning to myself to loosen prose to indicate that it should point to a description, citation, or copy of the source.

[Martin Holmes]

Council agreed to implement 2014-06-30.

[James Cummings]

rephrased at [r12991]