doc: remove extraneous paragraph from assert doc

The stability index is explained elsewhere in the documentation. It is
not necessary to repeat the information about Locked stability index in
the assert documentation.

PR-URL: https://github.com/nodejs/node/pull/11174


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>