[lic++][docs] Explain noexcept policy for narrow contracts.

Adds documentation around libc++'s policy to add noexcept to things that cannot throw but aren't marked as noexcept.

Refs LWG 3518 and D95251.

Differential Revision: https://reviews.llvm.org/D95821

GitOrigin-RevId: fab194898b5ed79d458813885af1187f57bb0a33
2 files changed