doc: simplify "is recommended" language in assert documentation

Replace "X is not recommended" with  "Avoid X".
Replace "It is recommended not to use X" with "Avoid X".

PR-URL: https://github.com/nodejs/node/pull/30558


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>