Update arrays.md for element-wise assignment

[ghost]

When assigning a scalar to multiple array values, the .= operator is required. The Assignment section is updated to reflect this.

[mbauman]

Oh, wonderful catch, thank you! Yes, this needs to be changed. The tricky thing about describing this behavior in this section is that it depends upon the broadcasting shape of both sides of the .=, but I think what you have here is concise and accurate enough.

[mbauman]

Let's also invert the If-clause here and tighten up this description while we're at it. Maybe something like:

If any index I_k selects more than one location, then the right hand side X must be an array with the same shape* as the result of indexing A[I_1, I_2, ..., I_n] or a vector with the same number of elements.

This could lead to a slightly different wording than what you proposed:

The dotted assignment operator .= can be used to broadcast the assignment of fewer elements in X across a compatible shape selected by the indices [I_1, I_2, …, I_n].

What do you think? Edit: I actually think what you first wrote about broadcasting might be better.

* That's still not quite true as we also allow adding or removing singleton dimensions to make the shapes match, but this is definitely more accurate than what was there before and I find that extra condition quite confusing to explain.

[KristofferC]

Can we merge this as an incremental improvement? @mbauman

[mbauman]

Definitely an improvement! Thanks @anders-dc.