Must not be a bad idea to improve documents. [ci skip]
In fact many functions declared in the header file are already
documented more or less. They were just copy & pasted, with applying
some style updates.
* bitwise operation between different enumeration types
('ruby_value_type' and 'ruby_fl_type') is deprecated
[-Wdeprecated-enum-enum-conversion]
* volatile-qualified parameter type 'volatile int' is deprecated
[-Wdeprecated-volatile]
