Added test for check doc strings in crates.

#1856

crates/ra_syntax/src/parsing/lexer.rs

@@ -1,3 +1,5 @@
+//! FIXME: write short doc here
+
 use crate::{
     SyntaxKind::{self, *},
     TextUnit,

crates/ra_syntax/src/parsing/text_token_source.rs

@@ -1,3 +1,5 @@
+//! FIXME: write short doc here
+
 use ra_parser::Token as PToken;
 use ra_parser::TokenSource;
 

crates/ra_syntax/src/parsing/text_tree_sink.rs

@@ -1,3 +1,5 @@
+//! FIXME: write short doc here
+
 use std::mem;
 
 use ra_parser::{ParseError, TreeSink};