Fix typos and wording.

doc/extending/op.txt

@@ -272,13 +272,12 @@ Optional methods or attributes
 
    If you define this function then it will be used instead of C code
    or perform() to do the computation while debugging (currently
-   DebugMode, but others amy also use it in the future).  It has the
+   DebugMode, but others may also use it in the future).  It has the
    same signature and contract as :func:`perform`.
 
-   This enables ops that don't have a deterministic output for
-   efficiency reasons to make their output determinisitic or do fix
-   problems that they encounter with DebugMode.  If your op doesn't
-   have any problems, don't implement this.
+   This enables ops that cause trouble with DebugMode with their
+   normal behaviour to adopt a different one when run under that
+   mode. If your op doesn't have any problems, don't implement this.
 
 If you want your op to work with gradient.grad() you also need to
 implement the functions described below.