Replace all occurrences of 'when' with 'if' in documentation and tutorial.

Also update the naturaldocs keywords file.

Closes #1396

doc/keywords.txt

@@ -17,4 +17,4 @@ self str syntax
 tag true type
 u16 u32 u64 u8 uint unchecked unsafe use
 vec
-when while with
+while with

doc/rust.texi

@@ -3331,12 +3331,12 @@ let message = alt x @{
 
 Finally, alt patterns can accept @emph{pattern guards} to further refine the
 criteria for matching a case. Pattern guards appear after the pattern and
-consist of a bool-typed expression following the @emph{when} keyword. A pattern
+consist of a bool-typed expression following the @emph{if} keyword. A pattern
 guard may refer to the variables bound within the pattern they follow.
 
 @example
 let message = alt maybe_digit @{
-  some(x) when x < 10 @{ process_digit(x) @}
+  some(x) if x < 10 @{ process_digit(x) @}
   some(x) @{ process_other(x) @}
 @}
 @end example

doc/tutorial/control.md

@@ -67,7 +67,7 @@ that `(float, float)` is a tuple of two floats:
 
     fn angle(vec: (float, float)) -> float {
         alt vec {
-          (0f, y) when y < 0f { 1.5 * std::math::pi }
+          (0f, y) if y < 0f { 1.5 * std::math::pi }
           (0f, y) { 0.5 * std::math::pi }
           (x, y) { std::math::atan(y / x) }
         }