a_strex.c: improve documentation of 'tag2nbyte' lookup table

The 'tag2nbyte' lookup table maps the tags of ASN1 string types
to their respective character widths. It is used for example by
ASN1_STRING_to_UTF8(). This commit adds the tag names as comments.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6062)

crypto/asn1/a_strex.c

@@ -281,12 +281,22 @@ static int do_dump(unsigned long lflags, char_io *io_ch, void *arg,
 static const signed char tag2nbyte[] = {
     -1, -1, -1, -1, -1,         /* 0-4 */
     -1, -1, -1, -1, -1,         /* 5-9 */
-    -1, -1, 0, -1,              /* 10-13 */
-    -1, -1, -1, -1,             /* 15-17 */
-    1, 1, 1,                    /* 18-20 */
-    -1, 1, 1, 1,                /* 21-24 */
-    -1, 1, -1,                  /* 25-27 */
-    4, -1, 2                    /* 28-30 */
+    -1, -1,                     /* 10-11 */
+     0,                         /* 12 V_ASN1_UTF8STRING */
+    -1, -1, -1, -1, -1,         /* 13-17 */
+     1,                         /* 18 V_ASN1_NUMERICSTRING */
+     1,                         /* 19 V_ASN1_PRINTABLESTRING */
+     1,                         /* 20 V_ASN1_T61STRING */
+    -1,                         /* 21 */
+     1,                         /* 22 V_ASN1_IA5STRING */
+     1,                         /* 23 V_ASN1_UTCTIME */
+     1,                         /* 24 V_ASN1_GENERALIZEDTIME */
+    -1,                         /* 25 */
+     1,                         /* 26 V_ASN1_ISO64STRING */
+    -1,                         /* 27 */
+     4,                         /* 28 V_ASN1_UNIVERSALSTRING */
+    -1,                         /* 29 */
+     2                          /* 30 V_ASN1_BMPSTRING */
 };
 
 /*