Write migration guide for the content in custom/text

doc/migration/3.7/index.rst

@@ -1,6 +1,39 @@
 Migrating From Version 3.6 to 3.7
 =================================
 
+Text Module (``custom/text``)
+-----------------------------
+
+Using ``content`` to specify the text of the module is deprecated in favor of
+using the same concepts as all other modules (formats and labels).
+
+For example, the following text module:
+
+.. code-block:: dosini
+
+  [module/text]
+  type = custom/text
+  content = Hello World
+  content-foreground = #ff0000
+
+Should now look like this:
+
+.. code-block:: dosini
+
+  [module/text]
+  type = custom/text
+  label = Hello World
+  label-foreground = #ff0000
+
+Because it is set to its default value, the ``format`` setting can also be
+completely left out.
+
+In general, all properties on ``content`` also apply the same on ``label``
+(e.g. ``background``, ``font``), except for ``offset``,
+``prefix``, ``suffix`` (and their sub-properties).
+Those three properties have to instead be applied to ``format`` (e.g.
+``format-offset``).
+
 System Tray
 -----------