Add a comment around the set_{min,max}_version logic.

It's not immediately obvious what's going on here.

Change-Id: Ibbba80a6ff9ace4d88e89a42efc270b77b4fad12
Reviewed-on: https://boringssl-review.googlesource.com/14368
Reviewed-by: Steven Valdez <svaldez@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@chromium.org>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 88d5f29..2405306 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -924,6 +924,7 @@
 
 static int set_min_version(const SSL_PROTOCOL_METHOD *method, uint16_t *out,
                            uint16_t version) {
+  /* Zero is interpreted as the default minimum version. */
   if (version == 0) {
     *out = method->min_version;
     return 1;
@@ -938,6 +939,7 @@
 
 static int set_max_version(const SSL_PROTOCOL_METHOD *method, uint16_t *out,
                            uint16_t version) {
+  /* Zero is interpreted as the default maximum version. */
   if (version == 0) {
     *out = method->max_version;
     /* TODO(svaldez): Enable TLS 1.3 by default once fully implemented. */