Making modifications to array fields, and adding documentation.

Modified array fields for cleaner nullability; e.g.,

```
union { null, array[string]} makeMeNull = null;
```

Turns into:

```
array[string] makeMeNull = [];
```

And, added docs for the read, genotype, and variant datatypes.
These fields are _somewhat_ alignment relevant, so I kept them in the alignment sections.