ARROW-6386: [C++][Documentation] Add documentation on interpretation of null slots

[bkietz]

Adds explicit not that null slots should be interpreted as an unknown.

The boolean and compare kernels do not currently implement this behavior:
And(null, false) -> null
Compare(GREATER_EQUAL, null, 0U) -> null

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@53384de). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #5231   +/-   ##
=========================================
  Coverage          ?   65.29%           
=========================================
  Files             ?      497           
  Lines             ?    67538           
  Branches          ?        0           
=========================================
  Hits              ?    44099           
  Misses            ?    23439           
  Partials          ?        0

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 53384de...666d182. Read the comment docs.

[wesm]

I'm not sure about this. The specification documents don't provide any guidance about computational semantics, only data representation. It is stated elsewhere (I believe?) that the values "underneath" null slots are not defined.

[wesm]

-1. The spec says "Array slots which are null are not required to have a particular value"; I do not think we need to say more on the matter