doc: Document builder init argument

fieldx/src/lib.rs

@@ -417,10 +417,18 @@
 //! - **`attributes`** (see the [section above](#attrs_family)) – builder struct attributes
 //! - **`attributes_impl`** - attributes of the struct implementation
 //! - **`into`** – force all builder setter methods to attempt automatic type conversion using `.into()` method
-//! - **`opt_in`** - struct-level only argument; with it only fields with explicit `builder` can get their values from the builder
 //!
 //!   With `into` the example above wouldn't need `String::from` and the call could look like this:
 //!   `.description("some description")`
+//! - **`opt_in`** - struct-level only argument; with it only fields with explicit `builder` can get their values from the builder
+//! - **`init`** - struct-level only argument; specifies identifier of the method to call to finish object initialization.
+//!
+//!   There are a couple of notes to take into account:
+//!
+//!   - the method is called on freshly created object right before it is returned back to builder caller
+//!   - it must take and return `self`: `fn post_build(mut self) { self.foo = "bar"; self }`
+//!   - for reference-counted structs the method is invoked before they're wrapped into corresponding container;
+//!     this allows for `mut self` and direct access to the fields without use of inner mutability
 //!
 //! ### **`rc`**
 //!
@@ -743,12 +751,12 @@
 //!
 //! ### **`builder`**
 //!
-//! **Type**: function
+//! **Type**: helper
 //!
 //! Mostly identical to the [struct-level `builder`](#builder). Field specifics are:
 //!
 //! - no `attributes_impl` and `opt_in` (consumed, but ignored)
-//! - string literal specifies setter method name if the builder type for this field
+//! - string literal specifies setter method name of the builder type for this field
 //! - `attributes` and `attributes_fn` are correspondingly applies to builder field and builder setter method
 //!
 //! Field level only argument: