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](https://ardupilot.org/dev/docs/AP_Periph-Parameters.html#gps1-rate-ms-gps-update-rate-in-milliseconds) and older versions.

Start with the GPS1_RATE_MS entry on the AP_Periph Parameters documentation page linked in the issue. Review the current wording around “should” and update it to distinguish configuring a GPS receiver, when possible, from any health or bad-message threshold; clarify that MAVLink-connected GPS receivers cannot be configured this way.

Written by the indexing model from the issue text.