add doc strings to use_trivial_contructor.rs

crates/ide-db/src/use_trivial_contructor.rs

@@ -1,6 +1,9 @@
+//! Functionality for generating trivial contructors
+
 use hir::StructKind;
 use syntax::ast;
 
+/// given a type return the trivial contructor (if one exists)
 pub fn use_trivial_constructor(
     db: &crate::RootDatabase,
     path: ast::Path,