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[doc] better explanation of accepting key format.

Co-authored-by: Rye Biesemeyer <[email protected]>

Author: mashhurs

docs/reference/keystore.md

@@ -136,7 +136,7 @@ bin/logstash-keystore add ES_USER ES_PWD
 When prompted, enter a value for each key.
 
 ::::{note}
-Key values are limited to ASCII characters. It can only accept digits, letters and optional `.` (dots) and/or `_` (underscores) symbols included between or after.
+Key values are limited to ASCII letters (`a`-`z`, `A`-`Z`), numbers (`0`-`9`), underscores (`_`), and dots (`.`); they must be at least one character long and cannot begin with a number.
 ::::
 
 

logstash-core/src/main/java/org/logstash/secret/SecretIdentifier.java

@@ -79,7 +79,7 @@ private static String validateWithTransform(final String key, final String partN
         }
 
         if (!KEY_PATTERN.matcher(key).matches()) {
-            logger.warn(String.format("Invalid %s key appeared in keystore. Please remove it as it cannot be used in the pipelines", key));
+            logger.warn(String.format("Invalid `%s` key appeared in keystore. Please remove it as it cannot be used in the pipelines", key));
         }
         return key.toLowerCase(Locale.US);
     }