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, OIDs are usually represented in source code as numeric
+identifiers, or B<NID>s.  OpenSSL has an internal table of OIDs that
+are generated when the library is built, and their corresponding NIDs
+are available as defined constants.  For the functions below, application
+code should treat all returned values -- OIDs, NIDs, 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,
@@ -123,6 +129,10 @@ exists for a particular algorithm). As a result they B<cannot> be encoded or
 decoded as part of ASN.1 structures. Applications can determine if there
 is a corresponding OBJECT IDENTIFIER by checking OBJ_length() is not zero.
 
+These functions cannot return B<const> because an B<ASN1_OBJECT> can
+represent both an internal, constant, OID and a dynamically-created one.
+The latter cannot be constant because it needs to be freed after use.
+
 =head1 EXAMPLES
 
 Create an object for B<commonName>:
@@ -139,6 +149,7 @@ 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);