You won't find the length field of an array in the Java doc APIs (as far as I know). It is just simply something you learn about when learning Java. Other than the length field, the only methods you can call on an array itself (not an element of the array, but the array itself) are the standard methods from the Object class.
As seetharaman venkatasamy pointed out, the length() method is a method of the String class, as well as the CharSequence class, StringBuffer class, and a number of other classes. (You can look them all up in the Java Doc Index.)
For some more discussion on Arrays, including their length, you can look at this recent thread.