doc: document switchover from measured bytes to chars after setEncoding

Checklist

Ref: https://github.com/nodejs/node/issues/6798

This is documentation PR to denote the change of measurement against the highWaterMark in non-object streams after .setEncoding() is called. It changes from bytes to character.

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

Affected core subsystem(s)

doc