dt-bindings: writing-schema: Add details on YAML text blocks

The YAML format has a couple of different forms for multi-line text
blocks which control allowed characters and handling of line-breaks.
Getting this wrong is a common review issue. Either a literal block is
used when there's no formatting needed or a folded/literal block is
not used when there is formatting to maintain.

Add some descriptions of the different forms to point folks to in
reviews.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20240918195130.2024205-2-robh@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>