Clarify use cases close to current tags

[larkvi]

Document the differences between our ODD's uses and exiting TEI P5 uses, and why, so that users can understand which to use, and members of the community can underrstand the differences in order to either 1. not accuse us of duplicating current tags or 2. suggest ways that the uses can be harmonized.

• legalActor type="recipient" vs. correspAction type="received"
• legalActor type="issuer" vs. correspAction type="sent"
• diploPart type="protocol"/"datatio"/"salutatio" vs. opener/dateLine/salute