reflect.DeepEqual: document that empty and nil are unequal for slices
reflect.DeepEqual: document that empty and nil are unequal for slices

Update issue 4133.
Added a sentence of documentation to call out the behavior.

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/6572051

dsriram/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang