Better document implicit_encoding/originated_encoding (Follow-up from "Proto: contract-compatible encodings for implicit and originated accounts")

The following discussion from !5222 (merged) should be addressed:

• @yrg started a discussion: (+1 comment)

  "compatible with" is maybe a bit fuzzy as it could be interpreted as a property stronger than what you have. (In my mind, "compatible with" means "equivalent"/"can subsume in any context".)