Yes, though I can't find anything in the Civilopedia either.
From my experience units heal the following amount per turn:
- 20 in your own cities
- 15 in friendly territory (I think this includes friendly city states, but I'm not sure)
- 5 if units are healing in enemy territory
- 10 elsewhere
As in earlier games naval units can't heal outside of friendly territory (though this can be enabled with certain promotions).
Other factors that affect unit healing include:
- The "God of Healing" Pantheon (+30 healing in or adjacent to your Holy Site districts)
- The "Medic" industrial era support unit (boosts healing of adjacent units)
- An Apostle promotion that lets them heal units in the same way as a Medic
- The "Dead Sea" natural wonder (units heal completely in one turn)
- The Great Scientist, "Abu al-Qasim al-Zahrawi" (passive healing + retirement effect)