proxy-authentication entry

proxy-authentication entry

[1] Clarify that credentials includes HTTP authentication (sideshowbarker著, ) <https://github.com/whatwg/fetch/commit/d84658ee3f97b0dafa3ff3e4a2d10dbf8e77c2d6>

[2] Doc: Definition of credentials unclear · Issue #612 · whatwg/fetch () <https://github.com/whatwg/fetch/issues/612>

[3] Clarify that credentials include HTTP auth by sideshowbarker · Pull Request #616 · whatwg/fetch () <https://github.com/whatwg/fetch/pull/616>