Fix title of User Manual page

Asciidoc uses the first line of a doc as the title, so you can't put
comments before the heading.

Author: terrynsun

docs/user/manual.adoc

@@ -1,5 +1,3 @@
-// IMPORTANT: the master copy of this document lives in the https://github.com/rust-analyzer/rust-analyzer repository
-
 = User Manual
 :toc: preamble
 :sectanchors:
@@ -8,6 +6,10 @@
 :source-highlighter: rouge
 :experimental:
 
+////
+IMPORTANT: the master copy of this document lives in the https://github.com/rust-analyzer/rust-analyzer repository
+////
+
 At its core, rust-analyzer is a *library* for semantic analysis of Rust code as it changes over time.
 This manual focuses on a specific usage of the library -- running it as part of a server that implements the
 https://microsoft.github.io/language-server-protocol/[Language Server Protocol] (LSP).