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::<()>() }
```

compiler/rustc_mir/src/monomorphize/collector.rs

@@ -59,9 +59,9 @@
 //!
 //! ### 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
+//! 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.
 //!