Inventory plugin: restore documentation for oauth_token

By completely removing any documentation for api_token / oauth_token,
commit 609df3e suddenly broke support
for reading the auth token from environment variables (which magically
happens to work based on plugin options documentation).

Restore the documentation bits by matching docs fragment. While at it,
also match the documented list of supported env variables which can be
used as auth token sources.

Fixes #315

plugins/inventory/digitalocean.py

@@ -30,6 +30,18 @@
         this should always be C(community.digitalocean.digitalocean).
     required: true
     choices: ['community.digitalocean.digitalocean']
+  oauth_token:
+    description:
+     - DigitalOcean OAuth token.
+     - Template expressions can be used in this field.
+    required: true
+    type: str
+    aliases: [ api_token ]
+    env:
+      - name: DO_API_TOKEN
+      - name: DO_API_KEY
+      - name: DO_OAUTH_TOKEN
+      - name: OAUTH_TOKEN
   attributes:
     description: >-
       Droplet attributes to add as host vars to each inventory host.