Clarify usage of logging set-level command in documentation

The command only works for existing loggers. To set the level for
all loggers one must change the log4j configuration.

Change-Id: I44c04a2fcae8d8c9e812e7de28cf4ccfd3c31b65

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index c651243..e3aa1b9 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -103,11 +103,17 @@
 
 To make debug logs visible in Gerrit's log file, debug logging must be
 enabled for the `com.googlesource.gerrit.plugins.hooks` package. This can be
-done by editing the `log4j.properties` file (requires a Gerrit restart) or
-by setting the log level at runtime with the ssh command:
+done by setting the log level at runtime with the ssh command:
 
 ```
   ssh -p 29418 user@gerrit gerrit logging set-level DEBUG com.googlesource.gerrit.plugins.hooks
 ```
 
+Note that setting the log level at runtime only works for loggers that
+have already been created. Loggers that get created after the level was
+set will still be created with the default level.
+
+To set the level for all loggers, it is necessary to do it by editing the
+`log4j.properties` file. This requires the Gerrit server to be restarted.
+
 [1]: ../../../Documentation/config-gerrit.html#gerrit.canonicalWebUrl