PEM_read_...: document that garbage and other PEM data is skipped

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15949)

doc/man3/PEM_read_bio_PrivateKey.pod

@@ -358,6 +358,10 @@ will be used.
 
 =head1 NOTES
 
+The PEM reading functions will skip any extraneous content or PEM data of
+a different type than they expect. This allows for example having a certificate
+(or multiple certificates) and a key in the PEM format in a single file.
+
 The old B<PrivateKey> write routines are retained for compatibility.
 New applications should write private keys using the
 PEM_write_bio_PKCS8PrivateKey() or PEM_write_PKCS8PrivateKey() routines