Add docs for Azure AD B2C tenant options

Add Azure `AUTHORITY_HOST` setting docs

docs/backends/azuread.rst

@@ -21,14 +21,28 @@ To enable OAuth2 support:
 
       SOCIAL_AUTH_AZUREAD_OAUTH2_RESOURCE = 'https://graph.microsoft.com/'
 
+- Add the backend to the authentication backends setting::
+
+      AUTHENTICATION_BACKENDS = (
+          ...
+          'social_core.backends.azuread_tenant.AzureADOAuth2',
+          ...
+      )
+
+- If you are using an authority host other than the default ``AZURE_PUBLIC_CLOUD`` (``'login.microsoftonline.com'``)
+  then you can override the default with the  ``AUTHORITY_HOST`` setting. A list of Azure authority hosts can be found
+  in the `Azure Authority Hosts`_ doc::
+
+      SOCIAL_AUTH_AZUREAD_OAUTH2_AUTHORITY_HOST = ''
+
 
 Tenant Support
 --------------
 
 If the app is linked to a specific tenant (vs the common tenant) it's
 possible to use a version of the backend with tenant support.
 
-*Note: The backend are split because of the needed cryptography dependencies which must be installed manually.*
+*Note: The backends are split because of the needed cryptography dependencies which must be installed manually.*
 
 To enable OAuth2 Tenant support:
 
@@ -52,7 +66,7 @@ To enable OAuth2 Tenant support:
 
   When using Microsoft Graph, the resource needed is::
 
-      SOCIAL_AUTH_AZUREAD_OAUTH2_RESOURCE = 'https://graph.microsoft.com/'
+      SOCIAL_AUTH_AZUREAD_TENANT_OAUTH2_RESOURCE = 'https://graph.microsoft.com/'
 
 - Add the backend to the authentication backends setting::
 
@@ -62,4 +76,61 @@ To enable OAuth2 Tenant support:
           ...
       )
 
+- If you are using an authority host other than the default ``AZURE_PUBLIC_CLOUD`` ('login.microsoftonline.com')
+  then you can override the default with the  ``AUTHORITY_HOST`` setting. The Azure authority hosts are listed
+  in the `Azure Authority Hosts`_ doc::
+
+      SOCIAL_AUTH_AZUREAD_TENANT_OAUTH2_AUTHORITY_HOST = ''
+
+B2C Tenant
+----------
+If the app needs custom business logic for authentication then use the Azure AD B2C tenant.
+
+To enable OAuth2 B2C Tenant support:
+
+- Fill in ``Client ID`` and ``Client Secret`` settings. These values can be
+  obtained easily as described in `Azure AD Application Registration`_ doc::
+
+      SOCIAL_AUTH_AZUREAD_B2C_TENANT_OAUTH2_KEY = ''
+      SOCIAL_AUTH_AZUREAD_B2C_TENANT_OAUTH2_SECRET = ''
+
+- Fill in the tenant id::
+
+      SOCIAL_AUTH_AZUREAD_B2C_TENANT_OAUTH2_TENANT_ID = ''
+
+- Fill in the B2C policy::
+
+      SOCIAL_AUTH_AZUREAD_B2C_OAUTH2_POLICY = ''
+
+The policy should start with `b2c_`. For more information see `Azure AD B2C User flows and custom policies overview`_ doc.
+
+- Also it's possible to define extra permissions with::
+
+      SOCIAL_AUTH_AZUREAD_B2C_TENANT_OAUTH2_RESOURCE = ''
+
+  This is the resource you would like to access after authentication succeeds.
+  Some of the possible values are: ``https://graph.windows.net`` or
+  ``https://<your Sharepoint site name>-my.sharepoint.com``.
+
+  When using Microsoft Graph, the resource needed is::
+
+      SOCIAL_AUTH_AZUREAD_B2C_TENANT_OAUTH2_RESOURCE = 'https://graph.microsoft.com/'
+
+- Add the backend to the authentication backends setting::
+
+      AUTHENTICATION_BACKENDS = (
+          ...
+          'social_core.backends.azuread_tenant.AzureADB2COAuth2',
+          ...
+      )
+
+- If you are using an authority host other than the default ``AZURE_PUBLIC_CLOUD`` ('login.microsoftonline.com')
+  then you can override the default with the  ``AUTHORITY_HOST`` setting. The Azure authority hosts are listed
+  in the `Azure Authority Hosts`_ doc::
+
+    SOCIAL_AUTH_AZUREAD_B2C_TENANT_OAUTH2_AUTHORITY_HOST = ''
+
 .. _Azure AD Application Registration: https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app
+.. _Azure AD B2C User flows and custom policies overview: https://docs.microsoft.com/en-us/azure/active-directory-b2c/user-flow-overview
+.. _Azure Authority Hosts: https://docs.microsoft.com/en-us/python/api/azure-identity/azure.identity.azureauthorityhosts?view=azure-python
+