Better comments in KEY_PART_INFO struct

sql/structs.h

@@ -70,7 +70,13 @@ typedef struct st_key_part_info {	/* Info about a key part */
   Field *field;
   uint	offset;				/* offset in record (from 0) */
   uint	null_offset;			/* Offset to null_bit in record */
-  uint16 length;			/* Length of key_part */
+  uint16 length;                        /* Length of keypart value in bytes */
+  /* 
+    Number of bytes required to store the keypart value. This may be
+    different from the "length" field as it also counts
+     - possible NULL-flag byte (see HA_KEY_NULL_LENGTH)
+     - possible HA_KEY_BLOB_LENGTH bytes needed to store actual value length.
+  */
   uint16 store_length;
   uint16 key_type;
   uint16 fieldnr;			/* Fieldnum in UNIREG */