[libcxx] Improve design documentation for the external-thread-library   configuration
[libcxx] Improve design documentation for the external-thread-library
  configuration

NFC.

Differential revision: https://reviews.llvm.org/D28610

Reviewers: EricWF

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@292108 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx