make clearer that GPS1_RATE_MS is to set the GPS settings (if possible) and not used as threshold for bad GPS messages

Bug Report

Issue details

The documentation of GPS1_RATE_MS contains the word 'should' which could be read as this being the minimum rate a GPS must send positions so that the AP doesn't mark it's data as unhealthy.

The doc should make clear that this value is used to configure a GPS Receiver to send at that rate, if possible (e.g. a GPS connected via MAVLink can not be configured so this PARAMETER is completely unused in that case. At least that's my understanding.)

Version dev and older versions.