Enforce minimum draw timeout and clarify natspec

[trmid]

The prize pool needs at least 2 draw periods since the last awarded draw to ensure there is enough time for the draw manager to award a draw; therefore the draw timeout must be at least 2 otherwise the prize pool will never award a draw and will shutdown immediately after the first draw period closes.

Also clarified the drawTimeout natspec to explicitly state that the value is equal to the number of draws that have passed since the last awarded draw.

[linear]

GEN-1768 [28] drawTimeoutAt is one draw early compared to what the natspec states

[github-actions]

LCOV of commit ba4b59d during Tests with 100% Coverage #562

Summary coverage rate:
  lines......: 99.8% (519 of 520 lines)
  functions..: 100.0% (80 of 80 functions)
  branches...: no data found

Files changed coverage rate:
                                             |Lines       |Functions  |Branches    
  Filename                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================
  src/PrizePool.sol                          |99.6%    232| 100%    46|    -      0

[github-actions]

LCOV of commit ddf87b3 during Tests with 100% Coverage #567

Summary coverage rate:
  lines......: 99.8% (530 of 531 lines)
  functions..: 100.0% (83 of 83 functions)
  branches...: no data found

Files changed coverage rate:
                                             |Lines       |Functions  |Branches    
  Filename                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================
  src/PrizePool.sol                          |99.6%    232| 100%    46|    -      0