I have heard the 90 days limit in Croatia is counted separately and am not sure if it is true. Where can I find the official description?
Staying in Croatia does indeed not count towards your Schengen stay.
Stays in Bulgaria, Croatia, Ireland, Romania, Cyprus and the United Kingdom shall not be taken into account as they are not (yet) part of the Schengen area without internal borders.
Croatia has its own 90 out of 180 rule, as can be read on this website of their Ministry of Foreign and European Affairs. Holder of a valid Schengen visa do not need to apply separately.
Note that Croatia intends to apply for joining the Schengen area in 2015. Depending on the outcome and how soon your journey is, this should be taken into account.
The HKSAR passport allows 90 days stay in Croatia, as per the HK Immigration website: http://www.immd.gov.hk/eng/service/travel_document/visa_free_access.html
Please also refer to this document for information specific to countries: http://www.immd.gov.hk/pdf/Full_List_of_Visa-free_Access_or_Visa-on-arrival_for_HKSAR_Passport_en.pdf