Add a reminder for a VRT_MINOR_VERSION bump

This is for the 4.1 ABI, since the next major release will have a
VRT_MAJOR_VERSION bump instead. The two functions may also change
semantics before the next 4.1.x, but will still be _new_ as far as
releases are concerned.

include/vrt.h

@@ -40,6 +40,9 @@
  *
  * changes to consider with next VRT_MAJOR_VERSION bump:
  * - cache_vrt.c: -> ssize_t VRT_CacheReqBody(VRT_CTX, size_t)
+ *
+ * changes to consider with next VRT_MINOR_VERSION bump for 4.1:
+ * - cache_vcl.c: -> VRT_ref_vcl, VRT_rel_vcl
  */
 
 #define VRT_MAJOR_VERSION	3U