Document ShadowMapSize configuration option in celestia.cfg

[ajtribick]

Thanks to #1330 I'm now aware that there is a ShadowMapSize configuration option in celestia.cfg. I'm not sure what would be a sensible value here to test this, so it would be good to have this documented and an example value provided (even if it the relevant line is commented out).

[375gnu]

I'm not satisfied with the current results of self shadowing so I'd prefer to keep this value "secret". You can provide any 2**n value here, the bigger it the bigger texture is and higher quality shadows you have. 2048 is a reasonable value for modern hardware.

[bird0cosmic]

I have not tried it because it is still an experimental feature.