Document postderef_qq support for ->$#* interpolation

[zmughal]

This was added in commit https://github.com/Perl/perl5/commit/ff25e5dbbad6ccf83f2e2a874a3c90294ea8cb47.

[jkeenan]

The English style of the changes is now good. But I'd like someone else to review the content of the changes, as I myself have never used this Perl syntax. @rjbs perhaps?

[khwilliamson]

This addition is needed, but it is misleading. The number returned is related to the array length, but it is one off. The number returned is the index of the highest element in the array. So if the array has 10 elements, 0..9, this returns 9. This needs to be clarified

[zmughal]

I have updated the text again with @khwilliamson's suggestions. I think I might need to do a rebase again?

[khwilliamson]

Unfortunately, use of the word 'last' is ambiguous. It can mean the previous, or the final. I suggest using 'highest' instead.

I don't think rebasing is required at this point.

[khwilliamson]

Thanks

This was merged via 365f3e0f038557f283dc48f11d9dd1d2eb0dfb8b