libpq: What can and cannot be bound? How to know?

For example with [NOTIFY][1]. The doc states:

> Payload: This must be specified as a simple string literal

Does that mean we cannot bind the payload?
I.e. the pseudo code:
```
conn.exec(bind(msg), "NOTIFY {} $1", conn.escapeName(channel));
```
is invalid? And I must use instead
```
conn.exec("NOTIFY {} {}", conn.escapeName(channel),
conn.escapeLiteral(msg))`?
```
I can try, of course, but could there be a obvious way to know what can and
cannot be bound, just from the doc?

That would make it easier to deal with SQL injection to be able to bind for
example.
And knowing what can be bound would be useful.

My $0.02. Thanks, --DD

[1]: https://www.postgresql.org/docs/current/sql-notify.html