doc: improve consistency in usage of NULL

- add backticks around use of NULL
- convert from null to NULL where we mean
  NULL

Signed-off-by: Michael Dawson <michael_dawson@ca.ibm.com>

PR-URL: https://github.com/nodejs/node/pull/32726


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Christopher Hiller <boneskull@boneskull.com>