document the incremental training option in the web api training service

doc/Grobid-service.md

@@ -751,6 +751,7 @@ Launch a training for a given model. The service return back a training token (a
 |           |                     |                      | type | optional | type of training, `full`, `holdout`, `split`, `nfold`, default is `split` |
 |           |                     |                      | ratio | optional | only considered for `split` training mode, give the ratio (number bewteen 0 and 1) of training and evaluation data when splitting the annotated data, default is `0.9` |
 |           |                     |                      | n | optional | only considered for `nfold` training mode, give the number of folds to be used, default is `10` |
+|           |                     |                      | `incremental` | optional | boolean indicating if the training should be incremental (`1`) starting from the existing model, or not (default, `0`) |
 
 The `type` of training indicates which training and evaluation mode should be used:
 - `full`: the whole available training data is used for training a model