Docstring for isnumeric (#104)

* docstring for isnumeric

* add isnumeric to docs

* change order of two docstrings

Author: mcabbott

docs/src/api.md

@@ -37,10 +37,13 @@ Optimisers.update!
 Optimisers.adjust(::Any, ::Real)
 ```
 
-Calling `Functors.@functor` on your model's layer types by default causes the
-optimiser to act on all suitable fields. To restrict this, define `trainable`:
+Calling `Functors.@functor` on your model's layer types by default causes
+these functions to recurse into all children, and ultimately optimise
+all `isnumeric` leaf nodes.
+To further restrict this by ignoring some fields of a layer type, define `trainable`:
 
 ```@docs
+Optimisers.isnumeric
 Optimisers.trainable
 ```
 

src/interface.jl

@@ -52,6 +52,15 @@ end
 # default all rules to first order calls
 apply!(o, state, x, dx, dxs...) = apply!(o, state, x, dx)
 
+"""
+    isnumeric(x) -> Bool
+
+Returns `true` on any parameter to be adjusted by Optimisers.jl,
+namely arrays of non-integer numbers. Returns `false` on all other types.
+
+Requires also that `Functors.isleaf(x) == true`, to focus on e.g. the
+parent of a transposed matrix, not the wrapper.
+"""
 isnumeric(x::AbstractArray{<:Number}) = isleaf(x)  # isleaf to allow for e.g. transposed shared weights
 isnumeric(x::AbstractArray{<:Integer}) = false
 isnumeric(x) = false