Clarify documentation of serialized session format
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 35b267a..3642162 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -9155,9 +9155,11 @@
* opaque session_id[32];
* opaque master[48]; // fixed length in the standard
* uint32 verify_result;
- * opaque peer_cert<0..2^24-1>; // length 0 means no peer cert
- * uint8_t peer_cert_digest_type;
- * opaque peer_cert_digest<0..2^8-1>;
+ * select (MBEDTLS_SSL_KEEP_PEER_CERTIFICATE) {
+ * case enabled: opaque peer_cert<0..2^24-1>; // length 0 means no cert
+ * case disabled: uint8_t peer_cert_digest_type;
+ * opaque peer_cert_digest<0..2^8-1>;
+ * }
* opaque ticket<0..2^24-1>; // length 0 means no ticket
* uint32 ticket_lifetime;
* uint8 mfl_code; // up to 255 according to standard