[clang-tidy] Fix the documentation for linuxkernel-must-use-errs.

Summary:
This changes ReleaseNotes.txt to have the first sentence of the full
documentation from linuxkernel-must-use-errs.rst.

This addresses a comment from the review of rL367071 in
https://reviews.llvm.org/D59963.

Reviewers: Eugene.Zelenko

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65343

llvm-svn: 367333