windows: document panics in LazyProc
windows: document panics in LazyProc

Some of LazyProc's methods will panic if the procedure cannot be
found. This patch documents that fact.

Change-Id: If446a61e001ae04eaba0ff08b512df4def8a00b7
Reviewed-on: https://go-review.googlesource.com/73450
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>

multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys,golang/sys