Clarify usage of WatchSet methods and behaviour of First/Last

[radeksimko]

The behaviour of First/Last methods can be verified using the following simple test:

type Job struct {
	ID string
}

func TestRepro(t *testing.T) {
	dbSchema := &memdb.DBSchema{
		Tables: map[string]*memdb.TableSchema{
			"jobs": {
				Name: "jobs",
				Indexes: map[string]*memdb.IndexSchema{
					"id": {
						Name:    "id",
						Unique:  true,
						Indexer: &memdb.StringFieldIndex{Field: "ID"},
					},
				},
			},
		},
	}

	db, err := memdb.NewMemDB(dbSchema)
	if err != nil {
		t.Fatal(err)
	}

	txn0 := db.Txn(false)

	txn1 := db.Txn(true)
	t.Cleanup(txn1.Abort)
	err = txn1.Insert("jobs", &Job{
		ID: "one",
	})
	if err != nil {
		t.Fatal(err)
	}
	txn1.Commit()

	obj, err := txn0.First("jobs", "id")
	if err != nil {
		t.Fatal(err)
	}
	t.Logf("received obj = %#v", obj)
}

go test ./internal/scheduler -run=TestRepro -v -count=1
=== RUN   TestRepro
    scheduler_test.go:59: received obj = <nil>
--- PASS: TestRepro (0.00s)
PASS
ok  	github.com/hashicorp/terraform-ls/internal/scheduler	0.315s

[banks]

LGTM, thanks for improving this Radek!