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

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, stream