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

NFC.

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

Reviewers: EricWF

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292108 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed