docs: create entry for getstakeversions in rpcapi

[dnldd]

Fixes #532

[davecgh]

This should be string. You can't send a *chainhash.Hash over RPC.

[davecgh]

$ dcrctl help getstakeversions
getstakeversions "hash" count

Returns the stake versions statistics.

Arguments:
1. hash  (string, required)  The start block hash.
2. count (numeric, required) The number of blocks that will be returned.

Result:
{
 "stakeversions": [{ (array of object) Array of stake versions per block.
  "hash": "value",   (string)          Hash of the block.
  "height": n,       (numeric)         Height of the block.
  "blockversion": n, (numeric)         The block version
  "stakeversion": n, (numeric)         The stake version of the block
  "votes": [{        (array of object) The version and bits of each vote in the block
   "version": n,     (numeric)         The version of the vote.
   "bits": n,        (numeric)         The bits assigned by the vote.
  },...],
 },...],
}

[davecgh]

As you can see you're missing the required count and the returned result isn't specified.

[dnldd]

updating the doc, will be converting the result section to the new format.