Update heap dump notes. DO NOT MERGE.

We brought over the VM change that removed SIGUSR1 as a way to cause
an hprof dump, but didn't bring the replacement command-line stuff
("am dumpheap" and the updated version of runhat).  This updates the
documentation to reflect the state of the world in gingerbread.

Bug 3110872.

Change-Id: Ia64190584a344995670e663636e8e2c0983fb29f