Docs: Initial documentation

Created an initial set of documentation which can also be built
using `build_docs.pl` without errors.

Original commit: elastic/x-pack-elasticsearch@1a3ccf4eb9

src/main/java/org/elasticsearch/shield/authc/esusers/tool/ESUsersTool.java

@@ -333,11 +333,7 @@ public class ESUsersTool extends CliTool {
         private static final CliToolConfig.Cmd CMD = cmd(NAME, Useradd.class).build();
 
         public static Command parse(Terminal terminal, CommandLine cli) {
-            if (cli.getArgs().length == 0) {
-                return exitCmd(ExitStatus.USAGE, terminal, "username is missing");
-            }
-
-            String username = cli.getArgs()[0];
+            String username = (cli.getArgs().length > 0) ? cli.getArgs()[0] : null;
             return new ListUsersAndRoles(terminal, username);
         }
 

src/main/resources/org/elasticsearch/shield/authc/esusers/tool/esusers.help

@@ -21,6 +21,10 @@ COMMANDS
 
     userdel    Removes an existing native user from the system
 
+    roles      Manage roles of a single user
+
+    list       List users and roles
+
 NOTES
 
     [*] For usage help on specific commands please type "security <command> -h"