Issue 9691: document void[] arrays.

Fixes: https://issues.dlang.org/show_bug.cgi?id=9691

Also, revamped the heading levels in `arrays.dd`, which were all over the map before.
> " Casting an array that contains pointers to $(D ubyte[]) may run the risk of the GC collecting live memory"

*Casting* an array will not change whether the GC scans the memory, this is determined by how it is *allocated*. The former would need some runtime call for casts, and I don't think this would pass Walter's scrutiny. I'd replace this with "Allocating an array that contains pointers $(D ubyte[]) ..."