This patch allows training cavalry champions at the stable and removes it from the Persian fortress, because only Persians can currently build stables.
For comparison, the elephant stable, available currently only to the Mauryas, trains all types of elephants and whilst the Mauryas can train their elephant champions at captured cart/ptol/sele fortress, elephants can't be trained at their own (maur) fortress.
It also makes the cavalry speed technologies researchable at the stable (in addition to the corral); for comparison, the Nisean horses technology can also be researched at the Seleucid fortress.