Document how the Element Call base URL developer option is used

The developer option passes its value through to the widget URL verbatim,
so it has to be the URL of the page that joins a room rather than the
deployment root. That is not obvious from the field, and hughns asked for
it to be written down in docs/ rather than expanded inline in the app,
because the path depends on how the deployment was hosted.

Part of #4475.

docs/element_call.md

@@ -0,0 +1,53 @@
+# Element Call
+
+<!--- TOC -->
+
+* [Overview](#overview)
+* [Setting a custom base URL](#setting-a-custom-base-url)
+* [Why the application does not add the room path itself](#why-the-application-does-not-add-the-room-path-itself)
+* [Where the value is read](#where-the-value-is-read)
+
+<!--- END -->
+
+## Overview
+
+Element X embeds a copy of [Element Call](https://github.com/element-hq/element-call) in the
+application assets and loads it from
+`https://appassets.androidplatform.net/element-call/index.html`. Calls therefore work out of the box
+with no extra deployment, and the embedded copy is the one every user gets.
+
+Developers can point the application at another Element Call deployment instead — a local
+development server, a staging deployment, or a self-hosted one — with the **Element Call base URL**
+developer option.
+
+## Setting a custom base URL
+
+The option lives in *Settings* → *Developer options* → *Element Call*, under "Element Call base
+URL". On a release build the *Developer options* entry is hidden until the version number at the
+bottom of *Settings* is tapped seven times. Leaving the field empty restores the embedded copy.
+
+The value is used verbatim as the widget base URL, so it must be the URL of the page that joins a
+room, not the URL of the deployment root. For the public deployments that means:
+
+| Deployment | Value to enter |
+|---|---|
+| `call.element.io` | `https://call.element.io/room` |
+| `call.element.dev` | `https://call.element.dev/room` |
+
+Entering `https://call.element.io` without the `/room` suffix loads the Element Call lobby rather
+than the room, so the call is never joined.
+
+## Why the application does not add the room path itself
+
+The path that joins a room is a property of the deployment, not of Element Call. Unpacking the
+Element Call release tarball behind a reverse proxy can put it at any path — `https://call.foo.tld`,
+`https://call.foo.tld/some/path/somewhere` — and a proxy that already maps a subdomain onto
+`https://1.2.3.4/room` would end up with the suffix twice. The application passes the value through
+unchanged so that every one of those deployments can be reached.
+
+## Where the value is read
+
+`AppDeveloperSettingsView` renders the field and sends
+`AppDeveloperSettingsEvent.SetCustomElementCallBaseUrl`, which `DefaultAppPreferencesStore` persists.
+`DefaultCallWidgetProvider.getWidget` reads it back and falls back to the embedded copy when it is
+unset.