WebView: clarify CookieManager docs

No change to logic, only documentation change. This adds some
clarifications to getCookie()/setCookie():

* getCookie() returns multiple cookies, setCookie() accepts only one
* This briefly describes getCookie()'s output format because it's fairly
  simple
* This summarizes setCookie()'s input format and provides an example,
  but directs the reader to the RFC spec for a full description
* Minor formatting changes, including editing a javadoc link which
  doesn't currently render correctly in kotlin docs (for
  ValueCallback#onReceiveValue)

Fixes: 175471135
Test: m offline-sdk-docs -j4
Change-Id: Id9052fa03584caae5ffc92ed1f6c4d2641d900ce