Clarify CookieManager RFC reference.

RFC2109 has been obsolete for a long time, and the docs aren't very
clear what exactly the RFC has to do with CookieManager; the RFC is
about HTTP and it's not immediately clear how this would apply to a Java
API.

Update the reference to the current cookie spec (and hyperlink it), and
clarify the text to explain that the HTTP header formats from the RFC
are the formats used to get/set set cookies.

Fixes: 143086151
Test: make ds-docs
Change-Id: I5e5838d3435b74516847b63e485fdd93810284aa