os: adjust wait6/waitid comment for netbsd

CL 431855 changed (*Process).blockUntilWaitable on netbsd to use wait6
again.

Update #48789

Change-Id: I948f5445a44ab2e82c02560480a2a244d2b5f473
Reviewed-on: https://go-review.googlesource.com/c/go/+/483396
Reviewed-by: Benny Siegert <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: tklauser

src/os/wait_unimp.go

@@ -3,8 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // aix, darwin, js/wasm, openbsd, solaris and wasip1/wasm don't implement
-// waitid/wait6. netbsd implements wait6, but that is causing test
-// failures, see issue #48789.
+// waitid/wait6.
 
 //go:build aix || darwin || (js && wasm) || openbsd || solaris || wasip1