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
b5e00e5c3d