Minor corrections for the RAND_DRBG API documentation

- added some explaining text to a sentence that lost its context.
- removed mention of per-ssl drbg
- fix whitespace errors
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5804)

doc/man7/RAND_DRBG.pod

@@ -37,8 +37,7 @@ Typical examples for such special use cases are the following:
 
 =item *
 
-You want to use your own private DRBG instances, similar to how it
-is currently done in the ssl library.
+You want to use your own private DRBG instances.
 Multiple DRBG instances which are accessed only by a single thread provide
 additional security (because their internal states are independent) and
 better scalability in multithreaded applications (because they don't need
@@ -144,10 +143,12 @@ together and are being used.
     +------------------+      +------------------------------------+
 
 
-
-The method L<RAND_DRBG_bytes(3)> is a convenience method wrapping the
-L<RAND_DRBG_generate(3)> function, which serves the actual request for
-random data.
+The usual way to obtain random bytes is to call RAND_bytes(...) or
+RAND_priv_bytes(...). These calls are roughly equivalent to calling
+RAND_DRBG_bytes(<public>, ...) and RAND_DRBG_bytes(<private>, ...),
+respectively. The method L<RAND_DRBG_bytes(3)> is a convenience method
+wrapping the L<RAND_DRBG_generate(3)> function, which serves the actual
+request for random data.
 
 =head1 RESEEDING