[GISEL][NFC] Fix comment for widenScalarToNextPow2

The docstring for this function incorrectly specified when a widening is
not performed. This patch adds the additional specification for what
happens when the type size is a power of two but it is less than MinSize.

Author: michaelmaitland

llvm/include/llvm/CodeGen/GlobalISel/LegalizerInfo.h

@@ -879,7 +879,8 @@ class LegalizeRuleSet {
   }
 
   /// Widen the scalar to the next power of two that is at least MinSize.
-  /// No effect if the type is not a scalar or is a power of two.
+  /// No effect if the type is a power of two, except if the type is smaller
+  /// than MinSize, or if the type is a vector type.
   LegalizeRuleSet &widenScalarToNextPow2(unsigned TypeIdx,
                                          unsigned MinSize = 0) {
     using namespace LegalityPredicates;