[Docs] Reword and fix registration docs

docs/admin/22_registration.rst

 Registration and user invitations
 =================================
 
-User invitations and registration can be used to allow e.g. guardians, that
-don't have an account managed by the school, to use AlekSIS.
+In addition to central management of user accounts, AlekSIS allows self-registration
+by users. Registration can be either fully open, or based on personal invitations.
 
+In a system handling ciritcal data, access control should be as tight as possible.
+However, there are scenarios where central account creation is not feasible, e.g.
+for optional guardian accounts. In such a scenario, the invitation system allows
+for processes like handing out invitation codes as a letter or through e-mail
+campaigns.
 
 Configuration
 -------------
@@ -16,15 +21,17 @@ Registration
 Registration can be enabled via the configuration interface in frontend.
 
 In the ``Authentication`` tab, click the checkbox ``Enable signup`` to enable
-signup for everyone.
+signup for everyone. A menu item will be added for public registration.
+
+.. warning::
+   Do not enable this feature unless you intend to run a public AlekSIS instance.
 
 User invitations
 ~~~~~~~~~~~~~~~~
 
 .. _core-user-invitations:
 
-User invitation can be enabled and configured via the configuration
-interface in frontend.
+In the same location as public registration, the invitation system can be enabled.
 
 * Authentication
 
@@ -33,7 +40,9 @@ interface in frontend.
   * Size of packets: Configure how many packets are generated, defaults to 3.
 
 By default, an invitation code looks like the following:
-``abcde-abcde-abcde``
+``abcde-abcde-abcde``.
+
+A menu item will become available for users to enter their invitation code.
 
 Usage
 -----
@@ -51,14 +60,23 @@ invitation code.
   :width: 100%
   :alt: Invitations page
 
+This mechanism allows for registration of entirely new persons that do not
+exist in the system, e.g. if perosnal details are not known in advance.
+
 Invite existing person
 ~~~~~~~~~~~~~~~~~~~~~~
 
-To invite an existing person, open the Person in AlekSIS an click ``Invite
+To invite an existing person, open the person in AlekSIS and click ``Invite
 user``.
 
-The invitation will be send to the persons email address.
+The invitation will be sent to the person's email address, and can only
+be used by this person. Upon registration, the new account will automatically
+be linked to the existing person.
 
 .. image:: ../_static/invite_existing.png
   :width: 100%
   :alt: Invite existing person
+
+.. note::
+   Before using this feature, make sure to read and understand
+   :ref:`core-concept-person`.

docs/user/01_registration.rst

@@ -14,26 +14,23 @@ AlekSIS account.
   :width: 100%
   :alt: Signup formular
 
-Verify email address
-~~~~~~~~~~~~~~~~~~~~
+If enabled, you have to verify your email address after signup. To do so, click
+on the link you recieved on the email address you entered in the signup form.
 
-If enabled, you have to verify your email address after signup.
-
-To do so, click on the link you recieved on the email address you entered in
-the signup form.
+.. note::
+   Normally, AlekSIS does not allow public registratio, and all accounts are
+   centrally managed by administrators. Public registration is an optional
+   feature that has to be enabled by adminsitrators after careful consideration.
 
 Using an invitation code
 ------------------------
 
 If you have an invitation code, click ``Accept invitation`` in the sidebar and
-enter it. You will be redirected to the signup formular.
+enter it. You will be redirected to the signup form.
 
 .. image:: ../_static/accept_invite.png
   :width: 100%
   :alt: Accept invitation
 
-Using an invitation link
-------------------------
-
 If you've recieved an invitation link (e.g. via email), clicking the link
-will redirect you to the signup form.
+will redirect you to the signup form automatically.