Document Locked scan policy field

Add the help entry for the Locked field.

Part of zaproxy/zaproxy#9138.

Signed-off-by: thc202 <thc202@gmail.com>

Author: thc202

addOns/help/src/main/javahelp/contents/ui/dialogs/scanpolicy.html

@@ -15,6 +15,9 @@ <H1>Scan Policy Dialog</H1>
 The category screens allow you to define the levels for every individual rule.<br>
 </p>
 
+<H3>Locked</H3>
+Locked policies do not allow the use of any other scan rules than the ones defined in their configuration, preventing the inadvertent use of other/new scan rules.
+
 <H3>Threshold</H3>
 This controls how likely ZAP is to report potential vulnerabilities.<br>
 <ul>