[release-branch.go1.1] effective_go.html: be more accepting in the guidelines for interface names

««« CL 9274043 / bbe324079abe
effective_go.html: be more accepting in the guidelines for interface names
Fixes #5421.

R=golang-dev, bradfitz, adg
CC=golang-dev
https://golang.org/cl/9274043
»»»

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9137045