Update INSTALL_FAILED_REPLACE_COULDNT_DELETE's comment

To make the comment of INSTALL_FAILED_REPLACE_COULDNT_DELETE easy to
be understood by the developers.

INSTALL_FAILED_REPLACE_COULDNT_DELETE is used to express the situation
that the new package replace the system package neither with
DELETE_SYSTEM_APP flag nor delete application for users.

Fixes: 149890172
Test: m docs
Change-Id: I57fe81845c81376cfa61224dd37579ade2223fa1