on 'primaryPreferred, clarify the situations when secondary read may occur

https://docs.mongodb.org/manual/reference/read-preference/#primaryPreferred

"In most situations, operations read from the primary member of the set. However, if the primary is unavailable, as is the case during failover situations, operations read from secondary members."

Could we clarify what other scenarios can make the primary "unavailable" e.g. when the primary is under heavy load, or provide a list of possible scenarios where the read could be diverted to secondaries.