Make Dropout docstring clear w.r.t. N-D dropout

[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

• Tests are added
• Entry in NEWS.md
• Documentation, if applicable
• Final review from @dhairyagandhi96 (for API changes).

[ToucheSir]

Alternative to the suggested edit which avoids the e.g. + i.e. combo:

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).

[DhairyaLGandhi]

bors r+

[bors]

Build succeeded:

• buildkite/flux-dot-jl