Dimensions of properties like the estimated coefficients in docu (R)

This issue might be relevant for R as well:

https://github.com/DoubleML/doubleml-for-py/issues/85

We should ...

- [ ] ... shortly explain the dimensions `all_coef` (etc.) in the documentation
- [ ] ... add column and row names to the dimensions of the object

In R we should think of the (currently private) `get__...`-methods in this context, for example:

https://github.com/DoubleML/doubleml-for-r/blob/8790659a9490c874491fd7de8cc9360b36fcb41b/R/double_ml.R#L784

Start with the linked get__... methods in R/double_ml.R and inspect how properties such as all_coef are documented and represented. Clarify the dimensions in the documentation and add row and column names to the relevant object dimensions; the issue does not identify specific documentation files or tests.

Written by the indexing model from the issue text.