Document that RubyVM::InstructionSequence methods are implementation and version dependent

Fixes [Bug #6785]

iseq.c

@@ -3456,7 +3456,9 @@ succ_index_lookup(const struct succ_index_table *sd, int x)
  *  Document-class: RubyVM::InstructionSequence
  *
  *  The InstructionSequence class represents a compiled sequence of
- *  instructions for the Ruby Virtual Machine.
+ *  instructions for the Ruby Virtual Machine. Not all implementations of Ruby
+ *  may implement this class, and for the implementations that implement it,
+ *  the methods defined and behavior of the methods can change in any version.
  *
  *  With it, you can get a handle to the instructions that make up a method or
  *  a proc, compile strings of Ruby code down to VM instructions, and