Document more details on array assignment

[mbauman]

Following #29167 I reviewed the array assignment section of the documentation:

• Incorporates the changes I suggested there
• More carefully explain scalar assignment (and the conversion that occurs)
• Lead with the shape of the indices instead of the shape of the value
• And as a minor stylistic thing, use unicode subscripts like Iₖ instead of I_k

[jmkuhn]

In general I don't have issues with displaying unicode characters but it appears Macs don't ship with any fonts that include latin subscript small letter k or n (the subscript numbers are fine). This could make it more difficult for some users to read the documentation.

[mbauman]

Bummer. I find I₁[i₁], I₂[i₂], ..., Iₙ[iₙ] far more readable than I_1[i_1], I_2[i_2], ..., I_n[i_n]. But thanks for reporting that — it is important. Viewing the subscript/superscript page on my iPhone, it looks like we're limited to: ₐₑᵢⱼₒᵣᵤᵥₓᵦᵧᵨᵩᵪ. We could use Iₓ for the arbitrary I_k, but we don't really have a good replacement for I_n. As if Unicode's dearth of subscript support wasn't enough…

This is why we cannot have nice things.

[JeffBezanson]

Are you planning to change back the subscripts, or should this be merged as-is?

[mbauman]

I'll change them back.