[doc] Disable EXTRACT_PRIVATE

It generates a lot not relevant documentation for helper methods

Author: Pawel Karczewski

doc/libpmemobj++.Doxyfile.in

@@ -491,7 +491,7 @@ EXTRACT_ALL            = NO
 # be included in the documentation.
 # The default value is: NO.
 
-EXTRACT_PRIVATE        = YES
+EXTRACT_PRIVATE        = NO
 
 # If the EXTRACT_PRIV_VIRTUAL tag is set to YES, documented private virtual
 # methods of a class will be included in the documentation.