azure: add SAS authentication option

doc/030_preparing_a_new_repo.rst

@@ -499,6 +499,17 @@ account name and key as follows:
     $ export AZURE_ACCOUNT_NAME=<ACCOUNT_NAME>
     $ export AZURE_ACCOUNT_KEY=<SECRET_KEY>
 
+or
+
+.. code-block:: console
+
+    $ export AZURE_ACCOUNT_NAME=<ACCOUNT_NAME>
+    $ export AZURE_ACCOUNT_SAS=<SAS_TOKEN>
+
+With the later form, ensure your ``SAS_TOKEN`` does not start with a leading
+``?``. If the generated token starts with a leading ``?`` it is safe to just
+delete the first character (the ``?``) before use.
+
 Afterwards you can initialize a repository in a container called ``foo`` in the
 root path like this:
 

doc/040_backup.rst

@@ -593,6 +593,7 @@ environment variables. The following lists these environment variables:
 
     AZURE_ACCOUNT_NAME                  Account name for Azure
     AZURE_ACCOUNT_KEY                   Account key for Azure
+    AZURE_ACCOUNT_SAS                   Shared access signatures (SAS) for Azure
 
     GOOGLE_PROJECT_ID                   Project ID for Google Cloud Storage
     GOOGLE_APPLICATION_CREDENTIALS      Application Credentials for Google Cloud Storage (e.g. $HOME/.config/gs-secret-restic-key.json)