[Docs] Add explanation for code snippets line width (#34796)

Add "Line width for code snippets is 76 characters" to  Java Language Formatting Guidelines

Author: wangzhenhui1992

CONTRIBUTING.md

@@ -159,6 +159,7 @@ Please follow these formatting guidelines:
 
 * Java indent is 4 spaces
 * Line width is 140 characters
+* Line width for code snippets that are included in the documentation (the ones surrounded by `// tag` and `// end` comments) is 76 characters
 * The rest is left to Java coding standards
 * Disable “auto-format on save” to prevent unnecessary format changes. This makes reviews much harder as it generates unnecessary formatting changes. If your IDE supports formatting only modified chunks that is fine to do.
 * Wildcard imports (`import foo.bar.baz.*`) are forbidden and will cause the build to fail. This can be done automatically by your IDE: