rustdoc: Add rustdocs

src/rustdoc/astsrv.rs

@@ -1,4 +1,11 @@
-#[doc = "Provides all access to AST-related, non-sendable info"];
+#[doc(
+    brief = "Provides all access to AST-related, non-sendable info",
+    desc =
+    "Rustdoc is intended to be parallel, and the rustc AST is filled \
+     with shared boxes. The AST service attempts to provide a single \
+     place to query AST-related information, shielding the rest of \
+     Rustdoc from its non-sendableness."
+)];
 
 import rustc::syntax::ast;
 import rustc::middle::ast_map;