Use wording similar to generate_n for the result of for_each_n.

Fixes n3554/n3554#62

Author: jaredhoberock

algorithms.html

@@ -406,7 +406,7 @@ <h1>For each</h1>
         </cxx-effects>
 
         <cxx-returns>
-          <code>first + n</code>.
+          <code>first + n</code> for non-negative values of <code>n</code> and <code>first</code> for negative values.
         </cxx-returns>
 
         <cxx-remarks>
@@ -434,7 +434,7 @@ <h1>For each</h1>
         </cxx-effects>
 
         <cxx-returns>
-          <code>first + n</code>.
+          <code>first + n</code> for non-negative values of <code>n</code> and <code>first</code> for negative values.
         </cxx-returns>
 
         <cxx-remarks>