Clarify how-to-edit-a-policy following review

Following @rapenne-s review:

* make clear that the first method is for the most common policies
* explain how priority works
* provide the user a way to modify the default editor
* some typos

Reading again the help text of `qubes-policy-editor` I made some
corrections.

user/how-to-guides/how-to-edit-a-policy.rst

@@ -11,12 +11,16 @@ There are three ways to edit a :ref:`policy <user/reference/glossary:policies>`:
 * with :program:`qubes-policy-editor`, a command-line text editor that will validate the file
   before saving it.
 
-In this how-to, we will restrict the clipboard policy to prevent an *untrusted* qube from pasting something in a *vault* qube.
+In this how-to, we will restrict the clipboard policy to prevent an *untrusted* qube from pasting something in a *vault* qube, **as an example**.
+
+.. _edit-a-policy-with-qubes-os-global-config:
 
 Edit a policy with :program:`Qubes OS Global Config`
 ----------------------------------------------------
 
-1. You need to go to the :guilabel:`Clipboard` tab.
+:program:`Qubes OS Global Config` allows you to edit the most common policies. Following our clipboard example:
+
+1. you need to go to the :guilabel:`Clipboard` tab.
 
 2. Under :guilabel:`Custom policy`, select :guilabel:`Add`
 
@@ -31,9 +35,9 @@ Edit a policy with :program:`Qubes Policy Editor`
 
 1. Open :program:`Qubes Policy Editor`
 
-2. You have to:
+2. You have to either:
 
-   * either open an existing file using :menuselection:`&File --> &Open`  or :kbd:`Ctrl` + :kbd:`O`
+   * open an existing file using :menuselection:`&File --> &Open`  or :kbd:`Ctrl` + :kbd:`O`
    * or create a new file using :menuselection:`&File --> &New` or :kbd:`Ctrl` + :kbd:`N`
 
      In that case, you need to choose a filename. The filename can only contain alphanumeric characters, underscores and hyphens. The common practice is to use a name like :file:`{30}-{user}.policy` where:
@@ -41,11 +45,11 @@ Edit a policy with :program:`Qubes Policy Editor`
      * :samp:`{30}` indicates the priority (i.e. the default policies start with ``90`` while the policies from :program:`Qubes OS Global Config` start with ``50``)
      * :samp:`{user}` could be any name
 
-     In order to change some clipboard policy, :file:`20-clipboard` could be a good name.
+     In order to override some clipboard policy, :file:`30-clipboard` could be a good name. Starting with ``20`` makes sure that the file will be read before any file starting with ``31`` or more, especially :file:`50-config-clipboard.policy`: this file is automatically created if you :ref`edit-a-policy-with-qubes-os-global-config`. If you want to create a policy that will never override the policies from :program:`Qubes OS Global Config`, use a name starting with a number between ``51`` and ``89``. The default policies from Qubes OS start with ``90``, so using a number equal or superior might b bee useless.
 
 3. Add a line to the file. In order to prevent the *untrusted* qube from pasting to the *vault* qube, the line should be:
 
-   .. code: text
+   .. code:: text
 
       qubes.ClipboardPaste *       untrusted       vault   deny
 
@@ -54,15 +58,21 @@ Edit a policy with :program:`Qubes Policy Editor`
 Edit a policy with :program:`qubes-policy-editor`
 -------------------------------------------------
 
-`qubes-policy-editor` is a command-line tool that ensures that the filename of the policy is correct and that the syntax is valid.
+:program:`qubes-policy-editor` is a command-line tool that ensures that the syntax of the policy is valid. You have to run it as root:
 
-You can invoke the program with any valid filename (only alphanumeric characters, underscores and hyphens), with or without the ``.policy`` extension:
+.. code:: console
 
-.. code: console
+   [root@dom0] # qubes-policy-editor
 
-   [root@dom0] # qubes-policy-editor 20-clipboard
+An editor will open; it will be your default editor if you have set the environment variable ``$EDITOR`` or ``$VISUAL``, otherwise it will fall back to :program:`vi`. After saving the file, :program:`qubes-policy-editor` will check the content and tell you if there is something wrong with the syntax.
 
-Your default editor will open. After saving the file, `qubes-policy-editor` will check the file and tell you if there is something wrong with the syntax.
+It will open :file:`30-user.policy` by default but you can invoke the program with any valid filename (only alphanumeric characters, underscores and hyphens), without the ``.policy`` extension:
+
+.. code:: console
+
+   [root@dom0] # qubes-policy-editor 30-clipboard
+
+In that case, :program:`qubes-policy-editor` will also check that the filename of the policy is correct.
 
 See also
 --------