test,doc: clarify `buf.indexOf(num)` input range

Hopefully clarify the behaviour of `buffer.indexOf()` and
`buffer.includes()` for numbers in that they will be
truncated to uint8s.

Add tests for that behaviour.

Fixes: https://github.com/nodejs/node/issues/7591
PR-URL: https://github.com/nodejs/node/pull/7611


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>