[Docs][Format] Clarify handling of Null values in REE encoding

[zagto]

In the Columnar format, the handling of null values in the REE type is not described properly.

• The null count of the parent array should be 0, since the parent has no validity bitmap (and there is a general assumption in arrow code that a non-zero null count means the standard validity bitmap is present)
• The document only indirectly describes how Null values are encoded

Thanks @tustvold for catching this!

[1] #13333 (comment)

Component(s)

Documentation, Format