Update Settings.yaml and settings.md to include units for nav_max_terrain_follow_alt

[daijoubu]

Added units to the description for nav_max_terrain_follow_alt

[sensei-hacker]

I think maybe you have to update settings.yaml, then settings.md is auto-generated? So that INAV and settings.md don't get out of sync.

[daijoubu]

Updated settings.yaml and regenerated settings.md with the Python utility to keep them in sync.