The writeFeatures method should always return a string

This PR fixes  #2102

All writeFeatures, writeFeature and writeGeometry methods now return a string. The respective methods such as writeFeaturesObject (JSON) and writeFeaturesNode (XML) have been marked api but not stable as yet.
I guess you don't need `@function` in that case. As I understand it we use `@function` as an indicator to the docs tool that this is a function. But it's not necessary in that case.