Proc class define before a first method

mrb_define_method setting mrb->proc_class for method proc
arity for cfunc is not implemented yet
Update documentation of `summary` property of mrbgem spec.
Quote required mrbgem spec property.
Merge pull request #2413 from take-cheeze/summary_doc

Update documentation of `summary` property of mrbgem spec.
Merge pull request #2399 from ksss/cfunc-proc

Proc class should be defined before a first method definition