Clarify input_scitype for Static models

oops

docs/src/adding_models_for_general_use.md

@@ -1,4 +1,4 @@
-# Adding Models for General Use 
+# Adding Models for General Use
 
 !!! note
 
@@ -975,7 +975,7 @@ also appears in the EvoTrees.jl package.
 Here "user-supplied data" is what the MLJ user supplies when
 constructing a machine, as in `machine(models, args...)`, which
 coincides with the arguments expected by `fit(model, verbosity,
-args...)` when `reformat` is not overloaded. 
+args...)` when `reformat` is not overloaded.
 
 Overloading `reformat` is permitted for any `Model`
 subtype, except for subtypes of `Static`. Here is a complete list of
@@ -992,7 +992,7 @@ responsibilities for such an implementation, for some
   serving as a data front-end for operations like `predict`. It must
   always hold that `reformat(model, args...)[1] = reformat(model,
   args[1])`.
-  
+
 The fallback is `reformat(model, args...) = args` (i.e., slurps provided data).
 
 *Important.* `reformat(model::SomeModelType, args...)` must always return a tuple, even if
@@ -1204,7 +1204,7 @@ Your document string must include the following components, in order:
 - A closing *"See also"* sentence which includes a `@ref` link to the raw model type (if you are wrapping one).
 
 
-## Unsupervised models
+## Unsupervised
 
 Unsupervised models implement the MLJ model interface in a very
 similar fashion. The main differences are:
@@ -1214,28 +1214,30 @@ similar fashion. The main differences are:
   although this is not a hard requirement. For example, a feature selection tool (wrapping
   some supervised model) might also include a target `y` as input. Furthermore, in the
   case of models that subtype `Static <: Unsupervised` (see also [Static
-  transformers](@ref) `fit` has no training arguments at all, but does not need to be
+  transformers](@ref)) `fit` has no training arguments at all, but does not need to be
   implemented as a fallback returns `(nothing, nothing, nothing)`.
 
 - A `transform` and/or `predict` method is implemented, and has the same signature as
   `predict` does in the supervised case, as in `MLJModelInterface.transform(model,
   fitresult, Xnew)`. However, it may only have one data argument `Xnew`, unless `model <:
-  Static`, in which case there is no restriction.  A use-case for `predict` is K-means 
-  `MLJModelInterface.predict(model, fitresult, Xnew)`. A use-case is
-  clustering that `predict`s labels and `transform`s 
-  input features into a space of lower dimension. See [Transformers
-  that also predict](@ref) for an example.
+  Static`, in which case there is no restriction.  A use-case for `predict` is K-means
+  clustering that `predict`s labels and `transform`s input features into a space of lower
+  dimension. See [Transformers that also predict](@ref) for an example.
 
-- The `target_scitype` trait continues to refer to the output of `predict`, if
-  implemented, while a trait, `output_scitype`, is for the output of `transform`.
+- The `target_scitype` refers to the output of `predict`, if implemented. A new trait,
+  `output_scitype`, is for the output of `transform`. Unless the model is `Static` (see
+  below) the trait `input_scitype` is for the single data argument of `transform` (and
+  `predict`, if implemented). If `fit` has more than one data argument, you must overload
+  the train `fit_data_scitype`, which bounds the allowed `data` passed to `fit(model,
+  verbosity, data...)` and will always be a `Tuple` type. 
 
 - An `inverse_transform` can be optionally implemented. The signature
   is the same as `transform`, as in
   `MLJModelInterface.inverse_transform(model, fitresult, Xout)`, which:
    - must make sense for any `Xout` for which `scitype(Xout) <:
-     output_scitype(SomeSupervisedModel)` (see below); and
+	 output_scitype(SomeSupervisedModel)` (see below); and
    - must return an object `Xin` satisfying `scitype(Xin) <:
-     input_scitype(SomeSupervisedModel)`.
+	 input_scitype(SomeSupervisedModel)`.
 
 For sample implementatations, see MLJ's [built-in
 transformers](https://github.com/JuliaAI/MLJModels.jl/blob/dev/src/builtins/Transformers.jl)
@@ -1245,8 +1247,16 @@ and the clustering models at
 
 ## Static models (models that do not generalize)
 
-See [Static transformers](@ref) for basic implementation of models that do not generalize
-to new data but do have hyperparameters. 
+A model type subtypes `Static <: Unsupervised` if it does not generalize to new data but
+nevertheless has hyperparameters. See [Static transformers](@ref) for examples. In the
+`Static` case, `transform` can have multiple arguments and `input_scitype` refers to the
+allowed scitype of the slurped data, *even if there is only a single argument.* For
+example, if the signature is `transform(static_model, X1, X2)`, then the allowed
+`input_scitype` might be `Tuple{Table(Continuous), Table(Continuous)}`; if the signature
+is `transform(static_model, X)`, the allowed `input_scitype` might be
+`Tuple{Table(Continous)}`. The other traits are as for regular `Unsupervised` models, as
+described above.
+
 
 ### Reporting byproducts of a static transformation