Changed the mechanism risk manager uses for managing auction surplus

The mechanism previously proposed has two flaws. First, it is hard to
implement and requires a lot of additional bookkeeping in the contract
given that multiple auctions can be open at the same time and all of
them can generate a surplus. Secondly, it might happen that the auction
gets opened for 0.0001 TBTC because the rest was filled by the surplus.
In this scenario, the auction value is much lower than the minimum tBTC
lot size and the auction length set by the governance could make not
sense.

docs/design.adoc

@@ -238,7 +238,16 @@ Which strategy is used is a governable parameter.
 
 In case signer bonds were purchased by a third party before the auction was
 fully filled, TBTC acquired by the risk manager from potential partial auction
-takes will be used in the future, reducing the amount of a next auction.
+takes will be used in the future, to purchase signer bonds once the accumulated
+surplus value allows for it. For example:
+
+* Liquidation of 1 TBTC deposit, auction opened for 1 TBTC and early closed
+  after being filled for 0.3 TBTC total. 0.3 TBTC goes to the risk manager.
+* Liquidation of 1 TBTC deposit, auction opened for 1 TBTC and early closed
+  after being filled for 0.8 TBTC total. 0.8 TBTC goes to the risk manager.
+* Liquidation of 1 TBTC deposit, there is 1.1 TBTC in the surplus, instead of
+  opening an auction, risk manager purchases signer bonds reducing the surplus
+  to 0.1 TBTC.
 
 == Upgradeability