os/exec: move platform-specific LookPath docs to a common comment
We have four different LookPath variations (unix, windows, plan9, wasm),
each with slightly different doc comments. Unify the documentation and
move it to a single, common LookPath.
Change-Id: I56bae57e80887a73ef0f6933258ee0a48dbccdcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/734320 Reviewed-by: Roland Shoemaker <roland@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>