The "length" attribute isn't a normal member field like you'd find in any other class. It's a feature of the Java language syntax. Code that
"reads" this field is compiled into bytecode that invokes a specific operation which fetches the length of an array.
Because it's not a real "member", it's not anywhere in the API Javadocs. Instead, you can read about it in any introductory Java book, or in the Java Language Specification
here.