Docs: Add note about which secure settings are valid (#25212)

This commit adds a note to the docs to clarify that only some settings
can be used with the keystore.

docs/reference/setup/secure-settings.asciidoc

@@ -8,6 +8,9 @@ tool to manage the settings in the keystore.
 
 NOTE: All commands here should be run as the user which will run elasticsearch.
 
+NOTE: Only some settings are designed to be read from the keystore. See
+documentation for each setting to see if it is supported as part of the keystore.
+
 [float]
 [[creating-keystore]]
 === Creating the keystore