[libgweather/doc-fixes] docs: Divide migration guide in two

[Emmanuele Bassi <ebassi src gnome org>]

commit bb07ce744638bc562c28d39a47ff8ce9b97211ab
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Wed Nov 17 22:33:45 2021 +0000

    docs: Divide migration guide in two
    
    We have one section that lists changes that can be performed while still
    using GWeather 3.x, and another section that outlines changes that can
    only be done when porting to GWeather 4.

 doc/migrating-3to4.md | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)
---
diff --git a/doc/migrating-3to4.md b/doc/migrating-3to4.md
index b525551f..7fe97c80 100644
--- a/doc/migrating-3to4.md
+++ b/doc/migrating-3to4.md
@@ -1,6 +1,6 @@
 Title: Migrating from GWeather 3.x to GWeather 4
 
-## Migrating from GWeather 3.x to GWeather 4
+# Migrating from GWeather 3.x to GWeather 4
 
 GWeather 4 is a new major version that breaks API and ABI compared with
 GWeather 3.
@@ -12,6 +12,11 @@ zone. Applications should provide their own UI, if needed, according to the
 best practices of the [GNOME human interface
 guidelines](https://developer.gnome.org/hig/).
 
+## Preparations
+
+Before migrating to GWeather 4, there are steps to follow in order to minimise
+the changes you have to implement once you switch.
+
 ### Stop using `gweather_location_get_children()`
 
 In order to iterate over the child locations of a [class@GWeather.Location],
@@ -46,6 +51,11 @@ method has been returning `NULL` since then.
 
 The Yahoo! provider was removed in libgweather 3.28.
 
+## Changes
+
+The following changes in GWeather 4 are incompatible with GWeather 3.x, and
+must be performed at the time of the port.
+
 ### `GWeatherLocation` is a GObject
 
 [class@GWeather.Location] has been promoted to a full `GObject` type. This