doc: add note about crypto/tls cert selection callback.
doc: add note about crypto/tls cert selection callback.

CC=golang-codereviews
https://codereview.appspot.com/123950043

webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh