Add the prediction_resistance flag to the documentation.

Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8678)

doc/man3/RAND_DRBG_reseed.pod

@@ -13,7 +13,8 @@ RAND_DRBG_set_reseed_defaults
  #include <openssl/rand_drbg.h>
 
  int RAND_DRBG_reseed(RAND_DRBG *drbg,
-                      const unsigned char *adin, size_t adinlen);
+                      const unsigned char *adin, size_t adinlen,
+                      int prediction_resistance);
 
  int RAND_DRBG_set_reseed_interval(RAND_DRBG *drbg,
                                    unsigned int interval);
@@ -37,6 +38,10 @@ and mixing in the specified additional data provided in the buffer B<adin>
 of length B<adinlen>.
 The additional data can be omitted by setting B<adin> to NULL and B<adinlen>
 to 0.
+An immediate reseeding from a live entropy source can be requested by setting
+the B<prediction_resistance> flag to 1.
+This feature is not implemented yet, so reseeding with prediction resistance
+requested will always fail.
 
 RAND_DRBG_set_reseed_interval()
 sets the reseed interval of the B<drbg>, which is the maximum allowed number