spirv-val: Fix comment for checking extended alignments (#4937)

* getBaseAlignment with roundUp true is called the "extended"
  by the Vulka spec.
* roundUp also affects matrix alignment, not just struct and array

source/val/validate_decorations.cpp

diff --git a/source/val/validate_decorations.cpp b/source/val/validate_decorations.cpp
index c5764ac..7505850 100644
--- a/source/val/validate_decorations.cpp
+++ b/source/val/validate_decorations.cpp
@@ -179,8 +179,9 @@
 }
 
 // Returns base alignment of struct member. If |roundUp| is true, also
-// ensure that structs and arrays are aligned at least to a multiple of 16
-// bytes.
+// ensure that structs, arrays, and matrices are aligned at least to a
+// multiple of 16 bytes.  (That is, when roundUp is true, this function
+// returns the *extended* alignment as it's called by the Vulkan spec.)
 uint32_t getBaseAlignment(uint32_t member_id, bool roundUp,
                           const LayoutConstraints& inherited,
                           MemberConstraints& constraints,