[safestack] Explain why tinfo at the end of the buffer

Reviewers: pcc, eugenis, vlad.tsyrklevich

Reviewed By: vlad.tsyrklevich

Subscribers: llvm-commits, jfb, #sanitizers

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D57863

llvm-svn: 353482