Improve documentation of BIO_FLAGS_BASE64_NO_NL flag.

Fixes #12491.
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NTomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17744)

doc/man3/BIO_f_base64.pod

@@ -21,13 +21,26 @@ any data read through it.
 
 Base64 BIOs do not support BIO_gets() or BIO_puts().
 
+For writing, output is by default divided to lines of length 64
+characters and there is always a newline at the end of output.
+
+For reading, first line should be at most 1024
+characters long. If it is longer then it is ignored completely.
+Other input lines can be of any length. There must be a newline
+at the end of input.
+
+This behavior can be changed with BIO_FLAGS_BASE64_NO_NL flag.
+
 BIO_flush() on a base64 BIO that is being written through is
 used to signal that no more data is to be encoded: this is used
 to flush the final block through the BIO.
 
-The flag BIO_FLAGS_BASE64_NO_NL can be set with BIO_set_flags()
-to encode the data all on one line or expect the data to be all
-on one line.
+The flag BIO_FLAGS_BASE64_NO_NL can be set with BIO_set_flags().
+For writing, it causes all data to be written on one line without
+newline at the end.
+For reading, it forces the decoder to process the data regardless
+of newlines. All newlines are ignored and the input does not need
+to contain any newline at all.
 
 =head1 NOTES