doc: wrap and punctuate YAML description text

stream.md was the only YAML in doc/api that was using `>` for line
continuation, and only used it for a few descriptions, most used the
same wrapping style the rest of the documentation does.

Also added punctuation and capitialization to a few description
sentences.

Fixes: https://github.com/nodejs/node/pull/25413#discussion_r246451003

PR-URL: https://github.com/nodejs/node/pull/25419
Fixes: https://github.com/nodejs/node/issues/25413


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>