Clarify ambiguity between 'streams' and 'files'.

We want to general guidelines for stream-level encapsulation to
apply to multiplexed as well as degenerate streams. 'Files' can
therefore specific to the mine-type discussion for unmultiplexed
Ogg physical streams.
Fix ambiguous RFC 2119 keywords.

These are normative whether capitalized or not. Capitalize the ones
which make sense as normative requirements, and reword the rest.
Fix mixed-case RFC 2119 keywords
Apply further fixes from Ron.
minor language tweak to RTP draft
updating the update draft
Lowered the smallest packet that the multi-stream encoder can encode

Limit now at 2*streams-1 and anything below that returns OPUS_BUFFER_TOO_SMALL
rather than OPUS_BAD_ARG
Takes into account a corner case with the multistream encoder

Considers the case where one stream would eat up all the remaining bytes,
including a 2-byte size that would then not leave enough for the even a PLC
packet in the next streams.
Fixes comments