#774 @cause in att.transcriptional probably doesn't belong

[Hugh A. Cayless]

The @cause attribute, defined in http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-att.transcriptional.html, is constrained by a value list that only applies to <retrace>, and it is only discussed in the guidelines under that element, even though its membership makes it available on, e.g. <add>. It would actually be nice to be able to note a reason for an addition, but this doesn't permit it. @cause should probably only be available on <retrace>.