docs: Add small intro to idmap examples

When reading the documentation, I didn't understand why this list
examples of things that fail without using the mount idmap feature.
It seems pretty pointless and I doubted if I was missing something,
until I finished the examples, the next section and saw the examples
revisited.  After that, it all made sense.

Let's add one small sentence before, so the reader knows where this is
going and why examples that don't might seem relevant are used.

Link: https://lore.kernel.org/r/20220429135748.481301-1-rodrigo@sdfg.com.ar


Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Rodrigo Campos <rodrigo@sdfg.com.ar>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>