include/linux/compiler-gcc.h: improve __visible documentation

#if GCC_VERSION >= 40600

/*

 * Tell the optimizer that something else uses this function or variable.

 * When used with Link Time Optimization, gcc can optimize away C functions or

 * variables which are referenced only from assembly code.  __visible tells the

 * optimizer that something else uses this function or variable, thus preventing

 * this.

 */

#define __visible	__attribute__((externally_visible))

#endif