docs(wopi): document public and callback base URLs

Clarify how OXICLOUD_WOPI_PUBLIC_BASE_URL and OXICLOUD_WOPI_BASE_URL
should be set when the office editor runs on a different hostname.

example.env

@@ -150,6 +150,16 @@ OXICLOUD_WOPI_ENABLED=false
 # Example for OnlyOffice: http://onlyoffice/hosting/discovery
 #OXICLOUD_WOPI_DISCOVERY_URL=
 
+# Public OxiCloud URL used by the browser for the WOPI host page.
+# Keep this on the OxiCloud hostname, even when OnlyOffice/Collabora runs elsewhere.
+# Example: https://cloud.example.com
+#OXICLOUD_WOPI_PUBLIC_BASE_URL=
+
+# Optional callback URL the editor uses to reach OxiCloud's /wopi/* endpoints.
+# Set this only when the editor uses a different internal address than the browser.
+# Example: http://oxicloud:8086
+#OXICLOUD_WOPI_BASE_URL=
+
 # Secret key for signing WOPI access tokens
 # Falls back to OXICLOUD_JWT_SECRET if not set
 #OXICLOUD_WOPI_SECRET=