Update User_documentation_en.md

[StCyr]

• Resolves: #
• Target version: main

Summary

Adds documentation how permissions are granted when sharing boards, based on what I have found here:

deck/lib/Service/PermissionService.php

Line 169 in ea4655b

public function userCan(array $acls, $permission, $userId = null) {

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

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