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>

Author: rata

Documentation/filesystems/idmappings.rst

@@ -369,6 +369,11 @@ kernel maps the caller's userspace id down into a kernel id according to the
 caller's idmapping and then maps that kernel id up according to the
 filesystem's idmapping.
 
+Let's see some examples with caller/filesystem idmapping but without mount
+idmappings. This will exhibit some problems we can hit. After that we will
+revisit/reconsider these examples, this time using mount idmappings, to see how
+they can solve the problems we observed before.
+
 Example 1
 ~~~~~~~~~