GDB Manual section 18.5.1

The above mention section mentions that one can issue commands such as
  set index-cache on
to control index caching. I'm using GDB 8.2.1 (the latest version released) but this command errors out
  Reading symbols from a.out...done.
  (gdb) set index-cache on
  No symbol "index" in current context.
  (gdb)            

Using 'help set' also does not seem to list index-cache.
Am I doing something wrong or is the documentation out of sync?
Thanks,Nick