Modify rules for default variability of pointed-to types.

Now, the pointed-to type is always uniform by default (if an explicit
rate qualifier isn't provided).  This rule is easier to remember and
seems to work well in more cases than the previous rule from 6d7ff7eba2.

docs/ispc.rst

@@ -1507,9 +1507,8 @@ As with other basic types, pointers can be both ``uniform`` and
 
 ** Like other types in ``ispc``, pointers are ``varying`` by default, if an
 explicit ``uniform`` qualifier isn't provided.  However, the default
-variability of the pointed-to type (``uniform`` or ``varying``) is the
-opposite of the pointer variability. ** This rule will be illustrated and
-explained in examples below.
+variability of the pointed-to type is uniform. ** This rule will be
+illustrated and explained in examples below.
 
 For example, the ``ptr`` variable in the code below is a varying pointer to
 ``uniform float`` values.  Each program instance has a separate pointer
@@ -1529,7 +1528,7 @@ qualifier:
 ::
 
     float f = 0;
-    float * uniform pf = &f;  // uniform pointer to a varying float
+    varying float * uniform pf = &f;  // uniform pointer to a varying float
     *pf = 1;
 
 The placement of the ``uniform`` qualifier to declare a ``uniform`` pointer
@@ -1546,7 +1545,7 @@ out with a separate location in memory for each program instance.)
 ::
 
     float a;
-    float * uniform pa = &a;
+    varying float * uniform pa = &a;
     *pa = programIndex;  // same as (a = programIndex)
     
 Also as in C, arrays are silently converted into pointers:
@@ -1554,8 +1553,8 @@ Also as in C, arrays are silently converted into pointers:
 ::
 
     float a[10] = { ... };
-    float * uniform pa = a;     // pointer to first element of a
-    float * uniform pb = a + 5; // pointer to 5th element of a
+    varying float * uniform pa = a;     // pointer to first element of a
+    varying float * uniform pb = a + 5; // pointer to 5th element of a
 
 Any pointer type can be explicitly typecast to another pointer type, as
 long as the source type isn't a ``varying`` pointer when the destination