cmd/compile: add comment for the context on mismatch import path

Follow up suggestion in CL 596396.

Updates #54542

Change-Id: I47bf66684bb8397dc1cfbc4479e2279e59a40cfb
Reviewed-on: https://go-review.googlesource.com/c/go/+/596515
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>

Author: cuonglm

src/cmd/compile/internal/noder/unified.go

@@ -404,6 +404,7 @@ func readPackage(pr *pkgReader, importpkg *types.Pkg, localStub bool) {
 		r := pr.newReader(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
 
 		pkg := r.pkg()
+		// This error can happen if "go tool compile" is called with wrong "-p" flag, see issue #54542.
 		if pkg != importpkg {
 			base.ErrorfAt(base.AutogeneratedPos, errors.BadImportPath, "mismatched import path, have %q (%p), want %q (%p)", pkg.Path, pkg, importpkg.Path, importpkg)
 			base.ErrorExit()