improved documentation for vector clock interfaces

Ignore-this: 5b2b942e6ac7030262f02c1190e74bcc

darcs-hash:20140920060559-c21fe-cf017cb6e990e1f1b6130b84f11ba6689420e229
added unmodifying merge to vector clock interface

Ignore-this: bc012ef1bbfa23f6560a3e589291108

darcs-hash:20140920060618-c21fe-518abe8bdfc4077fb8798ac05193c16f6d17781f