Merge #1490

1490: Make Dropout docstring clear w.r.t. N-D dropout r=DhairyaLGandhi a=darsnack

Make the docstring specify N-dimensional dropouts more clearly (i.e. so that people don't think we don't have `Dropout3d` etc.).

### PR Checklist

- [x] ~~Tests are added~~
- [x] ~~Entry in NEWS.md~~
- [x] Documentation, if applicable
- [x] ~~Final review from `@dhairyagandhi96` (for API changes).~~


Co-authored-by: Kyle Daruwalla <daruwalla@wisc.edu>
Co-authored-by: Kyle Daruwalla <daruwalla.k.public@icloud.com>

Author: bors[bot]

src/layers/normalise.jl

@@ -51,6 +51,10 @@ end
 
 Dropout layer. In the forward pass, apply the [`Flux.dropout`](@ref) function on the input.
 
+To apply dropout along certain dimension(s), specify the `dims` keyword.
+e.g. `Dropout(p; dims = 3)` will randomly zero out entire channels on WHCN input
+(also called 2D dropout).
+
 Does nothing to the input once [`Flux.testmode!`](@ref) is `true`.
 """
 mutable struct Dropout{F,D}
@@ -420,4 +424,4 @@ function Base.show(io::IO, l::GroupNorm)
   print(io, "GroupNorm($(join(size(l.β), ", "))")
   (l.λ == identity) || print(io, ", λ = $(l.λ)")
   print(io, ")")
-end
+end