gc: describe debugging flags
gc: describe debugging flags

The change to -m is the only one necessary
to close the issue.  The others are useful
to know about when debugging but shouldn't
be in the usage message since they may go
away or change at any time.

Fixes issue 2802.

R=lvd, rsc
CC=golang-dev
http://codereview.appspot.com/5606046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang