doc: increase Buffer.concat() documentation

This commit adds documentation for two edge cases in
Buffer.concat(). Those cases are:

- totalLength is specified, but is not an integer.
- The combined buffer length is greater than totalLength.

PR-URL: https://github.com/nodejs/node/pull/11845
Fixes: https://github.com/nodejs/node/issues/11605


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>