Avoid using a weird syntax for documentation

Following up 465bd972ec2.

If the actual implementation still resides in C, it should be
documented in C just like all other places.

numeric.c

@@ -6008,7 +6008,22 @@ rb_int_s_isqrt(VALUE self, VALUE num)
     }
 }
 
-/* :nodoc: */
+/*
+ * call-seq:
+ *   Integer.try_convert(object) -> object, integer, or nil
+ *
+ * If +object+ is an \Integer object, returns +object+.
+ *   Integer.try_convert(1) # => 1
+ *
+ * Otherwise if +object+ responds to <tt>:to_int</tt>,
+ * calls <tt>object.to_int</tt> and returns the result.
+ *   Integer.try_convert(1.25) # => 1
+ *
+ * Returns +nil+ if +object+ does not respond to <tt>:to_int</tt>
+ *   Integer.try_convert([]) # => nil
+ *
+ * Raises an exception unless <tt>object.to_int</tt> returns an \Integer object.
+ */
 static VALUE
 int_s_try_convert(VALUE self, VALUE num)
 {

numeric.rb

@@ -296,28 +296,6 @@ class Integer
   end
 end
 
-#  call-seq:
-#    Integer.try_convert(object) -> object, integer, or nil
-#
-#  If +object+ is an \Integer object, returns +object+.
-#    Integer.try_convert(1) # => 1
-#
-#  Otherwise if +object+ responds to <tt>:to_int</tt>,
-#  calls <tt>object.to_int</tt> and returns the result.
-#    Integer.try_convert(1.25) # => 1
-#
-#  Returns +nil+ if +object+ does not respond to <tt>:to_int</tt>
-#    Integer.try_convert([]) # => nil
-#
-#  Raises an exception unless <tt>object.to_int</tt> returns an \Integer object.
-#
-def Integer.try_convert(num)
-=begin
-  Primitive.attr! 'inline'
-  Primitive.cexpr! 'rb_check_integer_type(num)'
-=end
-end if false
-
 class Float
   #
   # call-seq: