Merge pull request ceph#60331 from MaxKellermann/CodingStyle_fwd

CodingStyle: allow C++ forward declarations

Reviewed-by: Greg Farnum <[email protected]>
Reviewed-by: Patrick Donnelly <[email protected]>
Reviewed-by: Ilya Dryomov <[email protected]>

Author: idryomov

CodingStyle

@@ -108,6 +108,12 @@ by section.
    portability since `#pragma once` is widely supported and is known
    to work on GCC and Clang.
 
+* Header Files -> Forward declarations:
+
+    Forward declarations of structs, unions, classes and enums can be
+    used to reduce header dependencies.  This speeds up compile times
+    because the compiler has to process less code.
+
 
 The following guidelines have not been followed in the legacy code,
 but are worth mentioning and should be followed strictly for new code: