Auto merge of #128321 - BatmanAoD:catch-unwind-doc-update, r=Mark-Simulacrum

Update `catch_unwind` doc comments for `c_unwind`

Updates `catch_unwind` doc comments to indicate that catching a foreign exception _will no longer_ be UB. Instead, there are two possible behaviors, though it is not specified which one an implementation will choose.

Nominated for t-lang to confirm that they are okay with making such a promise based on t-opsem FCP, or whether they would like to be included in the FCP.

Related: rust-lang/rust#74990, rust-lang/rust#115285, rust-lang/reference#1226

Author: bors