[NFC][libc++] Add missing EXPLICIT to pair and tuple synopsis

The constructors for std::pair and std::tuple have been made conditionally
explicit, however the synopsis in the headers do not reflect that.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366735 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed