Discuss @backDeployed attribute [SE-0376]

[tshortli]

Description

A section on the @backDeployed attribute (SE-0376) is missing from the book. The attribute's intended use is very narrow; it's intended primarily for use in ABI stable libraries that ship in OSes. Because of the narrow applicability I wouldn't normally consider the attribute a high priority for documentation in TSPL. However, it's been brought to my attention on the Swift forums that many developers have misconceptions about what @backDeployed is designed to accomplish and as a result they're trying to use it in their apps (to create poyfills for system APIs) and getting frustrated. It would be nice to have a section of TSPL that spells out the intended uses of @backDeployed and specifically addresses this misconception.

rdar://117591533