UMA_HISTOGRAM_SCALED_EXACT_LINEAR could use better documentation.

I should have caught some of this during my original review - apologies for not catching.

Some specifics:
  - Parameters are not well explained (e.g. sample/count/value_max/scale).
  - We should probably use same terminology between sample and value_max - e.g. sample+sample_max or value+value_max.
  - We should include some documentation in the .md docs.
  - I think a practical example of using this would be useful. Maybe make a macro specialization explicitly for logging KB?

Separately, unrelated to docs, it would be good if it worked with enums that use kMaxValue so the last parameter could be omitted. In fact, given there's no use of this currently - perhaps we can just make that version required?

Also, didn't we have a client who were interested in using this? What happened to them?