Clarify array initializers and give examples for complex situations

[jskeet]

(Brought up by email.)

In particular:

• Arrays of arrays
• Multi-dimensional arrays

[jskeet]

Also address this comment from #1554:

Jon to file a new issue around mapping between nullable and non-nullable arrays, see #1386, to also include conversions - and anything else folks think is missing

... either file a new issue, or fix in the same PR.

[jskeet]

Note that none of the examples of arrays have "genuinely trailing" question marks, e.g. string[]? array. That may be fine and deliberate, or it may need clarification.

[jskeet]

Created #1792 for the conversions part, just to make them separable.

[jskeet]

Moving to Pre-C# 9.0 - although we really would like to do this.