doc,stream: clarify 'data', pipe() and 'readable'

We stated that 'data' and pipe() are preferred over 'readable'.
This commit clarifies that 'data' and pipe() are easier to understand,
but 'readable' might result in increased throughput.

Fixes: https://github.com/nodejs/node/issues/11587
PR-URL: https://github.com/nodejs/node/pull/13432


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>