Clarify the expected return values of the pre_http_request filter

[johnbillion]

I've seen several instances where the pre_http_request filter is used, and its return value is set to something other than a WP_Error or an array that looks like an HTTP API response. Returning such values (for example, boolean true) results in unexpected behaviour, so the documentation should be updated to clarify this.

[DrewAPicture]

In 34509:

Docs: Improve the summary and description for the pre_http_request filter docs to better illustrate expected values.

The filter explicitly expects one of three passed value types:

• An array containing 'headers', 'body', 'response', 'cookies', and 'filename' elements
• A WP_Error instance
• Boolean false (to avoid short-circuiting the response)

Props johnbillion.
Fixes #33995.