Update book and docs

Author: BenFordTytherington

book/src/getting-started/2-our-first-cxx-qt-module.md

@@ -143,6 +143,13 @@ These functions then need to be implemented **outside** the bridge using `impl q
 {{#include ../../../examples/qml_minimal/rust/src/cxxqt_object.rs:book_rustobj_invokable_impl}}
 ```
 
+### Inlining the self receiver
+
+If an `extern "RustQt"` block contains exactly one `QObject`, the self type of methods can be inferred.
+For instance, in a block with multiple or no `QObject`s, a function like `foo(&self)` or `foo(self: Pin<&mut Self>)`
+would not compile, but will compile with the `Self` type set to that blocks `QObject`.
+This is how CXX [handles it](https://cxx.rs/extern-rust.html) (see the Methods heading).
+
 This setup is a bit unusual, as the type `qobject::MyObject` is actually defined in C++.
 However, it is still possible to add member functions to it in Rust and then expose them back to C++.
 This is the usual workflow for `QObject`s in CXX-Qt.

crates/cxx-qt-gen/src/parser/cxxqtdata.rs

@@ -68,6 +68,11 @@ impl ParsedCxxQtData {
         }
     }
 
+    /// Inline any `Self` types in the methods signatures with the Ident of a qobject passed in
+    ///
+    /// If there are unresolved methods in the list, but inline is false, it will error,
+    /// as the self inlining is only available if there is exactly one `QObject` in the block,
+    /// and this indicates that no inlining can be done, but some `Self` types were present.
     fn try_inline_self_types(
         inline: bool,
         type_to_inline: &Option<Ident>,