Update breaking changes doc with changes to geo_shape defaults

This commit updates the breaking changes doc to highlight the
impact of changing geo_shape defaults on timed index creation.

Author: nknize

docs/reference/migration/migrate_7_0/mappings.asciidoc

@@ -54,13 +54,17 @@ An error will now be thrown when unknown configuration options are provided
 to similarities. Such unknown parameters were ignored before.
 
 [float]
-==== deprecated `geo_shape` Prefix Tree indexing
+==== Changed default `geo_shape` indexing strategy
 
 `geo_shape` types now default to using a vector indexing approach based on Lucene's new
 `LatLonShape` field type. This indexes shapes as a triangular mesh instead of decomposing
-them into individual grid cells. To index using legacy prefix trees `recursive` or `term`
-strategy must be explicitly defined. Note that these strategies are now deprecated and will
-be removed in a future version.
+them into individual grid cells. To index using legacy prefix trees the `tree` parameter
+must be explicitly set to one of `quadtree` or `geohash`. Note that these strategies are
+now deprecated and will be removed in a future version.
+
+IMPORTANT NOTE: If using timed index creation from templates, the `geo_shape` mapping
+should also be changed in the template to explicitly define `tree` to one of `geohash`
+or `quadtree`. This will ensure compatibility with previously created indexes.
 
 [float]
 ==== deprecated `geo_shape` parameters