mrbconf.h: add `MRB_USE_INLINE_METHOD_CACHE` option description.
Currently it does not improve the performance (yet).
//#define MRB_NO_METHOD_CACHE
/* size of the method cache (need to be the power of 2) */
//#define MRB_METHOD_CACHE_SIZE (1<<8)
+//#define MRB_USE_INLINE_METHOD_CACHE
/* add -DMRB_USE_METHOD_T_STRUCT on machines that use higher bits of function pointers */
/* no MRB_USE_METHOD_T_STRUCT requires highest 2 bits of function pointers to be zero */
