advancedtls: Add notes about required vs. optional for FileWatcherOptions (#7165)

diff --git a/security/advancedtls/crl_provider.go b/security/advancedtls/crl_provider.go
index 6777be0..9a72ca6 100644
--- a/security/advancedtls/crl_provider.go
+++ b/security/advancedtls/crl_provider.go
@@ -90,9 +90,9 @@
 // FileWatcherOptions represents a data structure holding a configuration for
 // FileWatcherCRLProvider.
 type FileWatcherOptions struct {
-	CRLDirectory               string          // Path of the directory containing CRL files
-	RefreshDuration            time.Duration   // Time interval (default value is 1 hour) between CRLDirectory scans, can't be smaller than 1 minute
-	CRLReloadingFailedCallback func(err error) // Custom callback executed when a CRL file can’t be processed
+	CRLDirectory               string          // Required: Path of the directory containing CRL files
+	RefreshDuration            time.Duration   // Optional: Time interval (default 1 hour) between CRLDirectory scans, can't be smaller than 1 minute
+	CRLReloadingFailedCallback func(err error) // Optional: Custom callback executed when a CRL file can’t be processed
 }
 
 // FileWatcherCRLProvider implements the CRLProvider interface by periodically