[clang] [clang][docs] Add note about `-Wms-bitfield-padding` in MSVC compatibility docs (PR #155770)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

I wasn't sure if this was warranted, but given little effort to add the note I figured that I could create it an ping the last person to touch it :D

https://github.com/llvm/llvm-project/pull/155770