Clarify documentation of Image source ratio option.

[alvinlindstam]

Clarifies documentation for the ratio option of ImageWMS, ImageMapGuide, ImageVector and ImageCanvas. Should make it clear that the ratio should be at least 1, and that the ratio is in relation to width/height (not area).

Fixes #3926

[alvinlindstam]

All the Image sources with ratios use 1.5 as default, except MapGuideImage which uses 1. Is this intended?

[tschaub]

Thanks for the doc improvement @alvinlindstam.

I think 1.5 makes a sensible default ratio. The MapGuide source started out with an unconfigurable ratio of 1. This was later extracted as an option with the same default value. If there is a good reason to change it, it would make a pretty simple pull request.

[alvinlindstam]

Ok. 1.5 seems like a reasonable default. I was thinking of changing to 1.5 for MapGuide as well, for consistency but backwards comparability is more important.