AP_Motors: MOT_THST_HOVER param desc min/max fixed

[rmackay9]

This updates the MOT_THST_HOVER parameter descriptions to match the actual limits enforced in the code (see here)

This inconsistency came up in this discussion during 4.6 beta testing

This change has been tested by running, "./Tools/autotest/autotest.py build.Copter test.Copter.Parameters" and reviewing the output of the "buildlogs/apm.pdef.xml" file before and after.

BEFORE

      <param humanName="Thrust Hover Value" name="MOT_THST_HOVER" documentation="Motor thrust needed to hover expressed as a number from 0 to 1" user="Advanced">
        <field name="Range">0.2 0.8</field>
      </param>

AFTER

      <param humanName="Thrust Hover Value" name="MOT_THST_HOVER" documentation="Motor thrust needed to hover expressed as a number from 0 to 1" user="Advanced">
        <field name="Range">0.125 0.6875</field>
      </param>

[amilcarlucas]

Nice improvement

[peterbarker]

LGTM

[rmackay9]

thanks for the reviews, merged so we can't save time on the dev call

[rmackay9]

this is included in 4.6.0-beta5