Issue #17827: document codecs.encode and codecs.decode

Issue #17827: document codecs.encode and codecs.decode

- Merge from 3.3
- Added to What's New since these are more important in 3.x,
  as the bytes<->bytes and str<->str codecs don't fit the
  text model convenience methods in 3.x the way they did the
  basestring<->basestring methods in the 2.x text model
- Included under Library in Misc/NEWS for the same reason