Docs: Improve documentation for `WP_Http::processHeaders()`.

Document the `@return` value using hash notation.

Props marekdedic, valchovski, davidbaumwald, deepaklalwani, thimalw, SergeyBiryukov.
Fixes #48350.
Built from https://develop.svn.wordpress.org/trunk@48630


git-svn-id: https://core.svn.wordpress.org/trunk@48392 1a063a9b-81f0-0310-95a4-ce76da25c4cd