[stdlib] Remove all FIXMEs from doc comments
[stdlib] Remove all FIXMEs from doc comments

Fixes <rdar://problem/18457701>

Swift SVN r23007

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation