Add small note that get_variable can publicize its data files

Nothing elaborate, just laying down the ground rules for expected usage
and pointing out that it now works. The idea is not to give people
ideas, but to let projects which already have this use case, use it with
confidence.

docs/markdown/Dependencies.md

@@ -90,6 +90,28 @@ following will happen: If 'default_value' was provided that value will
 be returned, if 'default_value' was not provided then an error will be
 raised.
 
+## Dependencies that provide resource files
+
+Sometimes a dependency provides installable files which other projects then
+need to use. For example, wayland-protocols XML files.
+
+```meson
+foo_dep = dependency('foo')
+foo_datadir = foo_dep.get_variable('pkgdatadir')
+custom_target(
+    'foo-generated.c',
+    input: foo_datadir / 'prototype.xml',
+    output: 'foo-generated.c',
+    command: [generator, '@INPUT@', '@OUTPUT@']
+)
+```
+
+*Since 0.63.0* these actually work as expected, even when they come from a
+(well-formed) internal dependency. This only works when treating the files to
+be obtained as interchangeable with a system dependency -- e.g. only public
+files may be used, and leaving the directory pointed to by the dependency is
+not allowed.
+
 # Declaring your own
 
 You can declare your own dependency objects that can be used