[libc++] [docs] Mark P1865 as complete since 11.0 as it was implemented together with P1135. Fix synopses in <barrier> and <latch>.

It was implemented in commit 54fa9ecd3088508b05b0c5b5cb52da8a3c188655 ([libc++] Implementation of C++20's P1135R6 for libcxx).

GitOrigin-RevId: e2279c2350b83650a858a541ff566eccd69d38ef
3 files changed