[clang] [docs] Update the changes of C++20 Modules in clang15

Since clang15 is going to be branched in July 26, and C++ modules still
lack an update on ReleaseNotes. Although it is not complete yet, I think
it would be better to add one since we've done many works for C++20
Modules in clang15.

Differential Revision: https://reviews.llvm.org/D129138