internal: Include private definitions in generated rustdoc

rust-analyzer has handy prebuilt `cargo doc` output at
https://rust-lang.github.io/rust-analyzer/ide/

However, it doesn't include private definitions, which makes it less
useful when trying to learn unfamiliar parts of the codebase.

Instead, pass `--document-private-items` so the HTML includes
information on private types and modules too. rustdoc renders these
with a padlock icon, so it's still clear that they're private.

This change also exposes some more rustdoc warnings, which I've fixed.

crates/syntax/src/syntax_editor/mapping.rs

@@ -1,6 +1,6 @@
 //! Maps syntax elements through disjoint syntax nodes.
 //!
-//! [`SyntaxMappingBuilder`] should be used to create mappings to add to a [`SyntaxEditor`]
+//! [`SyntaxMappingBuilder`] should be used to create mappings to add to a `SyntaxEditor`
 
 use itertools::Itertools;
 use rustc_hash::FxHashMap;