Q:

Meaning "Colay with mSATA"

In a datasheet of a embedded device (this one in my case) what does it mean that the PCIe bus is conneceted to a socket that says "Full-size Mini-PCIe (colay mSATA)"

A:

Mini-PCIe and mSATA are two different standards but which use the same physical connector. However they are not intercompatible by default.
As far as I can tell, "colay" is probably an abbreviation for co-layout, or co-support, indicating that the slot simultaneously support both the Mini-PCIe and mSATA standards allowing both types of cards to be plugged in.
This is confirmed by reading the link you post to. In the table under "Internal I/O", it says "Mini-PCIe: 1 (full size, support mSATA)"

