cmd/internal/obj: adjust (*Link).AllPos comment in inl.go

cmd/internal/obj: adjust (*Link).AllPos comment in inl.go

AllPos truncates and overwrites its slice-storage input instead
of appending.  This makes that clear.

Change-Id: I81653ff49a4a7d14fe9446fd6620943f3b20bbd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/449478
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>