net/rpc: add test for issue 7689 (gob error should cause EOF)
Helpfully supplied by tommi.virtanen in issue 8173.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://codereview.appspot.com/151370043
math/big: fix doc comments

Fixes issue 8904.

TBR=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/148650043