Fix prefetch "trigger" value's documentation in ARM

Fix prefetch "trigger" value's documentation in ARM

For the prefetch "trigger" parameter ARM states that when a cache
record with a lower TTL value is encountered during query processing,
it is refreshed. But in reality, the record is refreshed when the TTL
value is lower or equal to the configured "trigger" value.

Fix the documentation to make it match with with the code.

(cherry picked from commit ef344b1f52ff531a713a26aa7fbdb7715a7aadcf)