static_call: Update API documentation

 *   DECLARE_STATIC_CALL(name, func);

 *   DEFINE_STATIC_CALL(name, func);

 *   DEFINE_STATIC_CALL_NULL(name, typename);

 *   DEFINE_STATIC_CALL_RET0(name, typename);

 *

 *   __static_call_return0;

 *

 *   static_call(name)(args...);

 *   static_call_cond(name)(args...);

 *   static_call_update(name, func);

 *   static_call_query(name);

 *

 *   EXPORT_STATIC_CALL{,_TRAMP}{,_GPL}()

 *

 * Usage example:

 *

 *   # Start with the following functions (with identical prototypes):

 *   To query which function is currently set to be called, use:

 *

 *   func = static_call_query(name);

 *

 *

 * DEFINE_STATIC_CALL_RET0 / __static_call_return0:

 *

 *   Just like how DEFINE_STATIC_CALL_NULL() / static_call_cond() optimize the

 *   conditional void function call, DEFINE_STATIC_CALL_RET0 /

 *   __static_call_return0 optimize the do nothing return 0 function.

 *

 *   This feature is strictly UB per the C standard (since it casts a function

 *   pointer to a different signature) and relies on the architecture ABI to

 *   make things work. In particular it relies on Caller Stack-cleanup and the

 *   whole return register being clobbered for short return values. All normal

 *   CDECL style ABIs conform.

 *

 *   In particular the x86_64 implementation replaces the 5 byte CALL

 *   instruction at the callsite with a 5 byte clear of the RAX register,

 *   completely eliding any function call overhead.

 *

 *   Notably argument setup is unconditional.

 *

 *

 * EXPORT_STATIC_CALL() vs EXPORT_STATIC_CALL_TRAMP():

 *

 *   The difference is that the _TRAMP variant tries to only export the

 *   trampoline with the result that a module can use static_call{,_cond}() but

 *   not static_call_update().

 *

 */

#include <linux/types.h>