Add snippets for canonicalization

docs/plugins/lastgenre.rst

@@ -48,14 +48,29 @@ works using a `tree of nested genre names`_, represented using `YAML`_, where th
 leaves of the tree represent the most specific genres.
 
 The most common way to use this would be with a custom whitelist containing only
-a desired subset of genres. For example, if the custom whitelist contains `rock`
-and `heavy metal` (which is a child node of `rock` in the default genre tree)
-then an item that has its genre specified as `viking metal` would actually be
-tagged as `heavy metal` because neither `viking metal` nor its parent
-`black metal` are in the whitelist. Considering that, it's not very useful to
-use the default whitelist (which contains about any genre contained in the tree)
-with canonicalization because nothing would ever be matched to a more generic
-node since all the specific subgenres are in the whitelist to begin with.
+a desired subset of genres. Consider for a example this minimal whitelist::
+
+    rock
+    heavy metal
+    pop
+    
+together with the default genre tree. Then an item that has its genre specified
+as `viking metal` would actually be tagged as `heavy metal` because neither
+`viking metal` nor its parent `black metal` are in the whitelist. It always
+tries to use the most specific genre that's available in the whitelist.
+
+The relevant subtree path in the default tree looks like this::
+
+    - rock:
+        - heavy metal:
+            - black metal:
+                - viking metal
+
+Considering that, it's not very useful to use the default whitelist (which
+contains about any genre contained in the tree) with canonicalization because
+nothing would ever be matched to a more generic node since all the specific
+subgenres are in the whitelist to begin with.
+
 
 .. _YAML: http://www.yaml.org/
 .. _tree of nested genre names: https://raw.githubusercontent.com/beetbox/beets/master/beetsplug/lastgenre/genres-tree.yaml