UPDATING: Change update procedure to use etcupdate(8) over mergemaster(8)

This commit aligns the steps in UPDATING with the steps from the
handbook which already prefers etcupdate(8). While here also remove a
dubious comment.

PR: 252417
Reviewed by: ceri
Approved by: philip (mentor), imp
Differential Revision: https://reviews.freebsd.org/D28062