docs: clarify afterRequest successful/failed reflect responseHandling rule (#3440)

[manwithacat]

Fixes #3440.

The htmx:afterRequest event docs described detail.successful and detail.failed purely as "true if 20x status code", but the actual code (at src/htmx.js:4849, 4893–4894) derives them from the matched htmx.config.responseHandling rule's error flag:

const shouldSwap = responseHandling.swap
let isError = !!responseHandling.error
// ... beforeSwap can override isError ...
responseInfo.failed = isError
responseInfo.successful = !isError

When users customize responseHandling to allow non-2xx responses to swap — for example with {"code":"...", "swap": true} and no error: true — those responses are also marked successful in htmx:afterRequest. The issue reporter was surprised by this. As @MichaelWest22 confirmed in the issue, the behavior is intentional ("If you override the default of what is defined as successful to swap then you would expect this to flow though"); the docs were just incomplete.

This PR rewords the descriptions of both detail.successful and detail.failed to:

1. Reference htmx.config.responseHandling as the source of truth.
2. Keep the "20x = successful" framing as a parenthetical "by default" note so existing readers' mental model is preserved.
3. Add an explicit note about the custom responseHandling case so future users with the same setup don't hit the same surprise.

Docs-only — targeting master per CONTRIBUTING.md. No code change.