docs: Documented new fields returned by getSkuDetails().

Per bug b/9221265, documented new price_amount_micros and
price_currency_code fields.

Doc is staged to:

http://asolovay.mtv:9265/google/play/billing/billing_reference.html#getSkuDetails

bug: 9221265
Change-Id: I06798692b3329e5e0d1fe3aaf215d9c97f2363c0