Stricter docs for WP_Http::processResponse()

[marekdedic]

I believe the function could better document the and return type. From a look at the source, maybe something like this:

@return array {
    @type string $headers
    @type string $body
}

+ comments obviously.

[takeshifurusato]

fixed the documents

[SergeyBiryukov]

In 47161:

Docs: Add more specific description for WP_Http::processResponse() return value.

Props marekdedic, takeshifurusato.
Fixes #48351.