Update docs for related model collections

[SamuelJennings]

Improved documentation regarding FormCollections with many-to-one relationships as per #110.

Please let me know if it is clear enough or if you want me to change anything.

[jrief]

Thanks. I will merge this in 1.5.1. When redefining the models, I first have to migrate the models on the documentation site.

[tuky]

Great findings and thx for the documentation. Maybe you could add a code comment to lines 226 and 245 a la # attribute name has to match the related_name (see note below)? that would make the reference even more clear.

[SamuelJennings]

Updated with additional code comments.

[tolland]

yes. Hit same problem