This changes the main documentation, doc strings, source code comments, and a
couple error messages in the test suite. In some cases the word was removed
or edited some other way to fix the grammar.

It should StreamWriter for Codecs.encode() and StreamReader for Codec.decode().

Patch by Nick Weinhold.

- clarified the distinction between text encodings and other codecs
- clarified relationship with builtin open and the io module
- consolidated documentation of error handlers into one section
- clarified type constraints of some behaviours
- added tests for some of the new statements in the docs

codecs.StreamReader returned incomplete data when were called after
readline() or read(size).  Based on patch by Amaury Forgeot d'Arc.

str.encode, bytes.decode and bytearray.decode now use an
internal API to throw LookupError for known non-text encodings,
rather than attempting the encoding or decoding operation and
then throwing a TypeError for an unexpected output type.

The latter mechanism remains in place for third party non-text
encodings.

Also, improve some docstring descriptions of the 'errors' parameter.

Closes #14015.