Document yaml[line-length] rule (#3653)

src/ansiblelint/rules/yaml.md

@@ -53,6 +53,7 @@ Some of the detailed error codes that you might see are:
 - `yaml[empty-lines]` - _too many blank lines (...> ...)_
 - `yaml[indentation]` - _Wrong indentation: expected ... but found ..._
 - `yaml[key-duplicates]` - _Duplication of key "..." in mapping_
+- `yaml[line-length]` - _Line too long (... > ... characters)_
 - `yaml[new-line-at-end-of-file]` - _No new line character at the end of file_
 - `yaml[octal-values]`: forbidden implicit or explicit [octal](#octals) value
 - `yaml[syntax]` - YAML syntax is broken
@@ -72,6 +73,11 @@ for it does check these. If for some reason, you do not want to follow our
 defaults, you can create a `.yamllint` file in your project and this will take
 precedence over our defaults.
 
+## Additional Information for Multiline Strings
+
+Adhering to yaml[line-length] rule, for writing multiline strings we recommend using Block Style Indicator: literal style indicated by a pipe (|) or folded style indicated by a right angle bracket (>), instead of escaping the newlines with backslashes.
+Reference [guide] for writing multiple line strings in yaml.
+
 ## Problematic code
 
 ```yaml
@@ -95,3 +101,4 @@ bar: ... # Correct comment indentation.
 [1.2.0]: https://yaml.org/spec/1.2.0/
 [1.2.2]: https://yaml.org/spec/1.2.2/
 [yaml specification]: https://yaml.org/
+[guide]: https://docs.ansible.com/ansible/latest/reference_appendices/YAMLSyntax.html#yaml-basics