doc: extra clarification of historySize option

History caching in the `readline` io is active only for terminal
interaction. Appropriate variables are initialized and relevant
`_addHistory()` function is called only if exposed `terminal` option
of `readline.createInterface()` is set `true` by user or internal
output check.

This clarification is useful to assure users there will be now wasted
overhead connected with history caching if `readline` is used not
for terminal interaction (e.g. for reading files line by line).

Particularly this fix is helpful after #6352 landing.

PR-URL: https://github.com/nodejs/node/pull/6397


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexander Makarenko <estliberitas@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>