Updated the FAQ

[Slick]

It's actually in there already. But you are right. This needs to be clarified and referenced in a few places because it is buried in the section for capturing planets. Thanks.:

7.2.9 Captured population (by any means) loses all of its racial attributes and is identical to yours except they retain their atmosphere breathing so can be used to populate your colonies appropriately AND they slowly get unhappy over time so you need to account for that so they don’t riot (riot police, upc, etc.).The positive happiness effect of 10 troops of any type will exactly cancel out the negative happiness of having alien population on a planet.