The `rdoc lib` command produces nearly the same output as `rake rdoc`.
The `rdoc lib` command also has the benefit of being standard, whereas
Rake subcommands can vary from project to project.  Since the purpose of
either command is to support local development, as opposed to generating
official docs for consumption by e.g. rubygems, the standard command
seems preferable.

Note that the `rdoc` command outputs to the doc/ directory by default,
so this commit also adds that directory to the plugin .gitignore file.