docs: Noted that purchaseToken is long and cannot be truncated.

Noted in reference and integration doc that the token for Google Play
purchases may be up to 1,000 chars long, and the entire token must
be returned (not abbreviated or truncated) to any method that needs
it.

See first comment for doc stage location.

bug: 19099521
Change-Id: I0a0f187293ea522c218f6c3287b4c36ee6725a90