available.packages(), in the bundled utils package, contains a variety of fields to filter, prioritise or expand the data returned. The examples in the documentation only illustrate different ways of using the available_packages_filters option, and doesn't actually include an example of available.packages() in use.
The documentation should:
1. Include examples of available.packages() being actually called;
2. Include examples of calls using parameters other than filter alteration.
(This is one of those issues where I could probably fix it myself, if someone can point me to an example of how one goes about most usefully contributing patches from outside the R-core team)