[alg.shift] Avoid undefined iterator arithmetic on ForwardIterator.

Avoid using the undefined concept BidirectionalIterator.

Author: jensmaurer

source/algorithms.tex

@@ -3578,7 +3578,7 @@
 
 \pnum
 \returns
-\tcode{first + (last - first) - n}
+\tcode{first + (last - first - n)}
 if \tcode{n} is positive and \tcode{n < last - first},
 otherwise \tcode{first} if \tcode{n} is positive, otherwise \tcode{last}.
 
@@ -3605,7 +3605,8 @@
 The type of \tcode{*first} shall satisfy
 the \tcode{MoveAssignable} requirements.
 \tcode{ForwardIterator} shall meet
-the \tcode{BidirectionalIterator} or \tcode{ValueSwappable} requirements.
+the requirements of a bidirectional iterator\iref{bidirectional.iterators} or
+the \tcode{ValueSwappable} requirements.
 
 \pnum
 \effects
@@ -3614,7 +3615,7 @@
 from position \tcode{first + i} into \tcode{position first + n + i}
 for each non-negative integer \tcode{i < (last - first) - n}.
 In the first overload case, if \tcode{ForwardIterator} satisfies
-the \tcode{BidirectionalIterator} requirements,
+the requirements of a bidirectional iterator,
 does so in order starting
 from \tcode{i = (last - first) - n - 1} and proceeding to \tcode{i = 0}.