Rollup merge of #83787 - digama0:patch-1, r=bjorn3

Monomorphization doc fix

Only public items are monomorphization roots. This can be confirmed by noting that this program compiles:
```rust
fn foo<T>() { if true { foo::<Option<T>>() } }
fn bar() { foo::<()>() }
```
See also the [zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Why.20are.20non.20public.20items.20monomorphization.20roots.3F).

compiler/rustc_mir/src/monomorphize/collector.rs

@@ -59,11 +59,15 @@
 //!
 //! ### Discovering roots
 //!
-//! The roots of the mono item graph correspond to the non-generic
+//! The roots of the mono item graph correspond to the public non-generic
 //! syntactic items in the source code. We find them by walking the HIR of the
-//! crate, and whenever we hit upon a function, method, or static item, we
-//! create a mono item consisting of the items DefId and, since we only
-//! consider non-generic items, an empty type-substitution set.
+//! crate, and whenever we hit upon a public function, method, or static item,
+//! we create a mono item consisting of the items DefId and, since we only
+//! consider non-generic items, an empty type-substitution set. (In eager
+//! collection mode, during incremental compilation, all non-generic functions
+//! are considered as roots, as well as when the `-Clink-dead-code` option is
+//! specified. Functions marked `#[no_mangle]` and functions called by inlinable
+//! functions also always act as roots.)
 //!
 //! ### Finding neighbor nodes
 //! Given a mono item node, we can discover neighbors by inspecting its