docs: Documented new IABv5 sub upgrade/downgrade and manual renewal

docs: Documented new IABv5 sub upgrade/downgrade and manual renewal

Documented new support for users upgrading/downgrading subscriptions
in the middle of a sub period, and for users manually renewing
before the subscription period ends. Also updated testing docs
to reflect that sandbox now supports testing subscriptions, and
documented new "autoRenewing" field in INAPP_PURCHASE_DATA.

See first comment for doc stage location.

bug: 18982828
bug: 18916814
bug: 18951608
bug: 18908756
Change-Id: I530b57d840ed5aeefe4c6a688a2e59aec41ec7d0