Clarify a comment.

Per suggestion during code review of previous patch.

Change-Id: I9bf88d3127f32036c1668ad03ffc491947a4f696
diff --git a/libdex/DexFile.c b/libdex/DexFile.c
index d4b64db..4f3bf56 100644
--- a/libdex/DexFile.c
+++ b/libdex/DexFile.c
@@ -68,6 +68,7 @@
     return NULL;
 }
 
+/* (documented in header) */
 const char* dexGetBoxedTypeDescriptor(PrimitiveType type) {
     switch (type) {
         case PRIM_VOID:    return NULL;