Rollup merge of #78556 - camelid:mir-opt-nrvo-docs, r=jyn514

Link to pass docs from NRVO module docs

It can be easy to miss that this is documented on the pass's struct if you are
looking at the module docs.

Cc https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/what.20is.20NRVO.3F

compiler/rustc_mir/src/transform/nrvo.rs

@@ -1,3 +1,5 @@
+//! See the docs for [`RenameReturnPlace`].
+
 use rustc_hir::Mutability;
 use rustc_index::bit_set::HybridBitSet;
 use rustc_middle::mir::visit::{MutVisitor, NonUseContext, PlaceContext, Visitor};