RELNOTES: Document recent file handle layout changes

Reviewed by: olce, rmacklem, imp, emaste
Differential Revision: https://reviews.freebsd.org/D48093

(cherry picked from commit e0b7a9b6bee16f4855b392c0941e76b7fef777a0)

This commit has been adapted for stable/14 (references of the relevant
commits and the date have been changed) and the commits that actually
have been MFCed so far (mention of tmpfs has been removed).