[DOC] Added document for rb_check_symbol() [ci skip]

symbol.c

@@ -957,6 +957,18 @@ rb_check_id(volatile VALUE *namep)
     return lookup_str_id(name);
 }
 
+/**
+ * Returns Symbol for the given name if it is interned already, or
+ * nil.
+ *
+ * \param namep   the pointer to the name object
+ * \return        the Symbol for *namep
+ * \pre           the object referred by \p namep must be a Symbol or
+ *                a String, or possible to convert with to_str method.
+ * \post          the object referred by \p namep is a Symbol or a
+ *                String if non-nil value is returned, or is a String
+ *                if nil is returned.
+ */
 VALUE
 rb_check_symbol(volatile VALUE *namep)
 {