Added module doc comment to explain the purpose of the completion.
-//! FIXME: write short doc here
+//! Completion for associated items in a trait implementation.
+//!
+//! This module adds the completion items related to implementing associated
+//! items within a `impl Trait for Struct` block. The current context node
+//! must be within either a `FN_DEF`, `TYPE_ALIAS_DEF`, or `CONST_DEF` node
+//! and an direct child of an `IMPL_BLOCK`.
+//!
+//! # Examples
+//!
+//! Considering the following trait `impl`:
+//!
+//! ```ignore
+//! trait SomeTrait {
+//! fn foo();
+//! }
+//!
+//! impl SomeTrait for () {
+//! fn f<|>
+//! }
+//! ```
+//!
+//! may result in the completion of the following method:
+//!
+//! ```ignore
+//! # trait SomeTrait {
+//! # fn foo();
+//! # }
+//!
+//! impl SomeTrait for () {
+//! fn foo() {}<|>
+//! }
+//! ```
use hir::{self, Docs, HasSource};
use ra_assists::utils::get_missing_impl_items;
