[Bug-tar] state what degree of gzip compression the user will get

[Dan Jacobson]

We all know gzip can be run with -1, -9 --best, etc.
But do the tar docs divulge what degree of compression tar will use?
No. Maybe it just calls plain gzip, which then means it uses -6.

So document what gzip degree of compression the user will end up with.

At present all one can find is:
      About corrupted compressed archives: `gzip''ed files have no
      redundancy, for maximum compression.