Small clean-ups

- Comment for Countemove pruning -> Continuation history
- Fix comment in input_slice.h
- Shorter lines in Makefile
- Comment for scale factor
- Fix comment for pinners in see_ge()
- Change Thread.id() signature to size_t
- Trailing space in reprosearch.sh
- Add Douglas Matos Gomes to the AUTHORS file
- Introduce comment for undo_null_move()
- Use Stockfish coding style for export_net()
- Change date in AUTHORS file

closes https://github.com/official-stockfish/Stockfish/pull/3416

No functional change

src/position.cpp

@@ -988,7 +988,7 @@ void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Squ
 }
 
 
-/// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
+/// Position::do_null_move() is used to do a "null move": it flips
 /// the side to move without executing any move on the board.
 
 void Position::do_null_move(StateInfo& newSt) {
@@ -1027,6 +1027,9 @@ void Position::do_null_move(StateInfo& newSt) {
   assert(pos_is_ok());
 }
 
+
+/// Position::undo_null_move() must be used to undo a "null move"
+
 void Position::undo_null_move() {
 
   assert(!checkers());
@@ -1092,8 +1095,8 @@ bool Position::see_ge(Move m, Value threshold) const {
       if (!(stmAttackers = attackers & pieces(stm)))
           break;
 
-      // Don't allow pinned pieces to attack (except the king) as long as
-      // there are pinners on their original square.
+      // Don't allow pinned pieces to attack as long as there are
+      // pinners on their original square.
       if (pinners(~stm) & occupied)
           stmAttackers &= ~blockers_for_king(stm);