protocol docs: clarify mempool.get_fee_histogram

closes spesmilo#209

docs/protocol-methods.rst

@@ -766,12 +766,20 @@ pool, weighted by transaction size.
   Fee intervals may have variable size.  The choice of appropriate
   intervals is currently not part of the protocol.
 
-**Example Result**
+  *fee* uses sat/vbyte as unit, and must be a non-negative integer or float.
 
-  ::
+  *vsize* uses vbyte as unit, and must be a non-negative integer.
+
+**Example Results**
+
+::
 
     [[12, 128812], [4, 92524], [2, 6478638], [1, 22890421]]
 
+::
+
+   [[59.5, 30324], [40.1, 34305], [35.0, 38459], [29.3, 41270], [27.0, 45167], [24.3, 53512], [22.9, 53488], [21.8, 70279], [20.0, 65328], [18.2, 72180], [18.1, 5254], [18.0, 191579], [16.5, 103640], [15.7, 106715], [15.1, 141776], [14.0, 183261], [13.5, 166496], [11.8, 166050], [11.1, 242436], [9.2, 184043], [7.1, 202137], [5.2, 222011], [4.8, 344788], [4.6, 17101], [4.5, 1696864], [4.1, 598001], [4.0, 32688687], [3.9, 505192], [3.8, 38417], [3.7, 2944970], [3.3, 693364], [3.2, 726373], [3.1, 308878], [3.0, 11884957], [2.6, 996967], [2.3, 822802], [2.2, 9075547], [2.1, 12149801], [2.0, 16387874], [1.4, 873120], [1.3, 3493364], [1.1, 2302460], [1.0, 23204633]]
+
 
 server.add_peer
 ===============