NFC: Fix -Wdocumentation warning

Author: davezarzycki

include/swift/AST/Attr.h

@@ -1321,7 +1321,7 @@ class ImplementsAttr : public DeclAttribute {
   }
 };
 
-/// A limited variant of \c @objc that's used for classes with generic ancestry.
+/// A limited variant of \c \@objc that's used for classes with generic ancestry.
 class ObjCRuntimeNameAttr : public DeclAttribute {
   static StringRef getSimpleName(const ObjCAttr &Original) {
     assert(Original.hasName());

include/swift/AST/Decl.h

@@ -2528,7 +2528,7 @@ class ValueDecl : public Decl {
   ///     value.getFormalAccessScope().getDeclContext())</code>
   ///
   /// If \p treatUsableFromInlineAsPublic is true, declarations marked with the
-  /// \c @usableFromInline attribute are treated as public. This is normally
+  /// \c \@usableFromInline attribute are treated as public. This is normally
   /// false for name lookup and other source language concerns, but true when
   /// computing the linkage of generated functions.
   ///

lib/Sema/LookupVisibleDecls.cpp

@@ -948,7 +948,7 @@ static void lookupVisibleMemberAndDynamicMemberDecls(
 /// Enumerates all keypath dynamic members of \c baseType, as seen from the
 /// context \c dc.
 ///
-/// If \c baseType is \c @dynamicMemberLookup, this looks up any keypath
+/// If \c baseType is \c \@dynamicMemberLookup, this looks up any keypath
 /// dynamic member subscripts and looks up the members of the keypath's root
 /// type.
 static void lookupVisibleDynamicMemberLookupDecls(