Fix comment about length of text, bytea, etc.

When commit 3e23b68dac introduced
single-byte varlena headers, it rendered this comment incomplete.

src/include/c.h

@@ -446,7 +446,7 @@ struct varlena
 /*
  * These widely-used datatypes are just a varlena header and the data bytes.
  * There is no terminating null or anything like that --- the data length is
- * always VARSIZE(ptr) - VARHDRSZ.
+ * always VARSIZE_ANY_EXHDR(ptr).
  */
 typedef struct varlena bytea;
 typedef struct varlena text;