array variables

I am writing a pl/pgsql function that needs to use an array. I can find precious little documentation of the use of arrays, especially in postgresql's official documentation. Is there any comprehensive documentation for plpgsql syntax (not just arrays but all areas)? For example, I have found code examples that use the _arrayvar syntax, but I want to know where these programmers figured this out as I can't find it on postgresql.org.

In the meanwhile, how do I declare an array of fixed length and then later reference it to assign a value to an array item?

James F.