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.

set `true` -> set to `true`