Minor DRY cleanup of CBL_Attachment initializer
Minor DRY cleanup of CBL_Attachment initializer

With commit 4f6e505, the "revpos" field was being read with identical
code in two places. Combined those into one. Also updated a few comments.

lordkev/couchbase-lite-ios,lordkev/couchbase-lite-ios,lordkev/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,pegli/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,amco/couchbase-lite-ios,couchbase/couchbase-lite-ios,brendand/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,brendand/couchbase-lite-ios,couchbase/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,pegli/couchbase-lite-ios,amco/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,amco/couchbase-lite-ios,tomco/couchbase-lite-ios,alfared/couchbase-lite-ios,amco/couchbase-lite-ios,tomco/couchbase-lite-ios,brendand/couchbase-lite-ios,couchbase/couchbase-lite-ios,lordkev/couchbase-lite-ios,couchbase/couchbase-lite-ios,brendand/couchbase-lite-ios,alfared/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,couchbase/couchbase-lite-ios,tomco/couchbase-lite-ios,pegli/couchbase-lite-ios,alfared/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,alfared/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,amco/couchbase-lite-ios,pegli/couchbase-lite-ios,couchbase/couchbase-lite-ios,brendand/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,amco/couchbase-lite-ios,tomco/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,amco/couchbase-lite-ios