[WebAssembly] Convert a file-level comment to doxygen style.

llvm-svn: 254996
diff --git a/llvm/lib/Target/WebAssembly/WebAssembly.td b/llvm/lib/Target/WebAssembly/WebAssembly.td
index 53dd929..551ad93 100644
--- a/llvm/lib/Target/WebAssembly/WebAssembly.td
+++ b/llvm/lib/Target/WebAssembly/WebAssembly.td
@@ -6,10 +6,11 @@
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
-//
-// This is a target description file for the WebAssembly architecture, which is
-// also known as "wasm".
-//
+///
+/// \file
+/// \brief This is a target description file for the WebAssembly architecture,
+/// which is also known as "wasm".
+///
 //===----------------------------------------------------------------------===//
 
 //===----------------------------------------------------------------------===//