doc: warn about using util.inspect/util.format

Because of the potential performance bottlenecks that may be introduced
by `util.inspect()` and `util.format()` in production hot path code.

Based on real user feedback, it is not obvious that these are intended
to be debugging tools.

PR-URL: https://github.com/nodejs/node/pull/17791


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>