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Update arrays.md for element-wise assignment #29167

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Update arrays.md for element-wise assignment #29167

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8 changes: 6 additions & 2 deletions doc/src/manual/arrays.md
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Expand Up @@ -424,8 +424,12 @@ subsections, and arrays of booleans to select elements at their `true` indices.

If `X` is an array, it must have the same number of elements as the product of the lengths of
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@mbauman mbauman Sep 13, 2018 •
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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.

the indices: `prod(length(I_1), length(I_2), ..., length(I_n))`. The value in location `I_1[i_1], I_2[i_2], ..., I_n[i_n]`
of `A` is overwritten with the value `X[i_1, i_2, ..., i_n]`. If `X` is not an array, its value
is written to all referenced locations of `A`.
of `A` is overwritten with the value `X[i_1, i_2, ..., i_n]`. If `X` is a scalar, use the
element-wise assignment operator `.=` to write the value to all referenced locations of `A`:
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```
A[I_1, I_2, ..., I_n] .= X
```

Just as in [Indexing](@ref man-array-indexing), the `end` keyword may be used
to represent the last index of each dimension within the indexing brackets, as
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