Auto merge of #4265 - vks:patch-1, r=flip1995
Auto merge of #4265 - vks:patch-1, r=flip1995

cast_ptr_alignment: Mention legal use under known problems

Refs #2881.

changelog: Mention know problems for cast_ptr_alignment

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust