dt-bindings: mtd: explicitly document nesting partitions descriptions

Documentation was already saying that fixed and dynamic partitioning can
be mixed but was missing a clear description and examples. This commit
adds a proper documentation of how descriptions can be nested and how
layouts can be mixed.

This addition is important for partitions that contain subpartitions.
In such cases partitions have to be properly described in order to let
system handle them correctly.

Depending on situation, nesting descriptions may provide more accurate
logic/structure and/or allow mixing partitioning types (various
"compatible" values).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>