add release note formally allowing run_target in an alias

We've now fixed it so it works, and it provides useful functionality,
e.g. creating a custom target that builds multiple gettext domains in
one action.

docs/markdown/snippets/run_target-depends.md

@@ -0,0 +1,20 @@
+## `run_target` can now be used as a dependency
+
+A `run_target()` can now be saved in a variable and reused as a dependency in
+an `alias_target()`. This can be used to create custom alias rules that ensure
+multiple other targets are run, even if those targets don't produce output
+files.
+
+For example:
+
+```
+i18n = import('i18n')
+
+all_pot_targets = []
+
+foo_i18n = i18n.gettext('foo')
+
+all_pot_targets += foo_i18n[1]
+
+alias_target('all-pot', all_pot_targets)
+```