doc: improve assert documentation

This fixes the officially accepted message types for `assert.throws()`,
`assert.rejects()`, `assert.doesNotThrow()` and
`assert.doesNotReject()`. It also renames the `block` argument in
those functions to `fn` and `promiseFn` for further clarity.

PR-URL: https://github.com/nodejs/node/pull/22692


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>