Change language used in instance_variable_set documentation [ci skip]

This uses less harsh language recommended by duerst.

Fixes [Misc #15265]
Fixes [Misc #15748]

object.c

@@ -2957,9 +2957,9 @@ rb_obj_ivar_get(VALUE obj, VALUE iv)
  *     obj.instance_variable_set(string, obj)    -> obj
  *
  *  Sets the instance variable named by <i>symbol</i> to the given
- *  object, thereby frustrating the efforts of the class's
- *  author to attempt to provide proper encapsulation. The variable
- *  does not have to exist prior to this call.
+ *  object. This may circumvent the the encapsulation intended by
+ *  the author of the class, so it should be used with care.
+ *  The variable does not have to exist prior to this call.
  *  If the instance variable name is passed as a string, that string
  *  is converted to a symbol.
  *