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 <[email protected]>
Reviewed-by: Krzysztof Kozlowski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Rob Herring (Arm) <[email protected]>

Author: robherring

Documentation/devicetree/bindings/writing-schema.rst

@@ -43,6 +43,36 @@ description
   or device does, standards the device conforms to, and links to datasheets for
   more information.
 
+  The YAML format has several options for defining the formatting of the text
+  block. The options are controlled with indicator characters following the key
+  (e.g. "description: \|"). The minimum formatting needed for a block should be
+  used. The formatting controls can not only affect whether the YAML can be
+  parsed correctly, but are important when the text blocks are rendered to
+  another form. The options are as follows.
+
+  The default without any indicators is flowed, plain scalar style where single
+  line breaks and leading whitespace are stripped. Paragraphs are delimited by
+  blank lines (i.e. double line break). This style cannot contain ": " in it as
+  it will be interpretted as a key. Any " #" sequence will be interpretted as
+  a comment. There's other restrictions on characters as well. Most
+  restrictions are on what the first character can be.
+
+  The second style is folded which is indicated by ">" character. In addition
+  to maintaining line breaks on double line breaks, the folded style also
+  maintains leading whitespace beyond indentation of the first line. The line
+  breaks on indented lines are also maintained.
+
+  The third style is literal which is indicated by "\|" character. The literal
+  style maintains all line breaks and whitespace (beyond indentation of the
+  first line).
+
+  The above is not a complete description of YAML text blocks. More details on
+  multi-line YAML text blocks can be found online:
+
+  https://yaml-multiline.info/
+
+  https://www.yaml.info/learn/quote.html
+
 select
   Optional. A json-schema used to match nodes for applying the
   schema. By default, without 'select', nodes are matched against their possible