RT2964: Fix it via doc

OBJ_nid2obj() and friends should be treated as const.
Reviewed-by: NDr. Stephen Henson <steve@openssl.org>

doc/crypto/OBJ_nid2obj.pod

@@ -45,6 +45,12 @@ Deprecated:
 
 The ASN1 object utility functions process ASN1_OBJECT structures which are
 a representation of the ASN1 OBJECT IDENTIFIER (OID) type.
+For convenience, OID's are usually represented in source code as numeric
+identifiers, or B<NID>'s.  OpenSSL has an internal table of OID's that
+are generated when the library is built, and their corresponding NID's
+are available as define'd constants.  For the functions below, application
+code should treat all returned values -- OID's, NID's, or names -- as
+constants.
 
 OBJ_nid2obj(), OBJ_nid2ln() and OBJ_nid2sn() convert the NID B<n> to
 an ASN1_OBJECT structure, its long name and its short name respectively,
@@ -139,6 +145,7 @@ Create a new NID and initialize an object from it:
 
  int new_nid;
  ASN1_OBJECT *obj;
+
  new_nid = OBJ_create("1.2.3.4", "NewOID", "New Object Identifier");
 
  obj = OBJ_nid2obj(new_nid);
@@ -156,6 +163,9 @@ Instead B<buf> must point to a valid buffer and B<buf_len> should
 be set to a positive value. A buffer length of 80 should be more
 than enough to handle any OID encountered in practice.
 
+Many of the functions here should probably be changed to return B<const>
+pointers. But the lack of consistency makes that too awkward to do.
+
 =head1 RETURN VALUES
 
 OBJ_nid2obj() returns an B<ASN1_OBJECT> structure or B<NULL> is an