[libc++][doc] Updates LWG3346 status. (#80536)

The issue addresses an obvious wording issue. Implementing the
constructors as specified in the synposis, as libc++ did, already
implements the fixed behaviour.

Updates:
- LWG3346 pair and tuple copy and move constructor have backwards
specification
GitOrigin-RevId: 04f99bec9af495c2571c9e7dcb14face3cfea4ce
1 file changed