gjolly and bradfitz websocket: add documentation to IsServerConn and IsClientConn
Those function names are ambiguous. These functions don't check if a
client/server is connected but if the connection is client or server
side. Adding some documentation should avoid any mistake.

Change-Id: I0522e176d58c558fd880d49b980b0152b96823bc
Reviewed-on: https://go-review.googlesource.com/120146
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Latest commit afe8f62 Jun 20, 2018