DOC: semiconnectedness

I couldn't find `is_semiconnected` in the online docs so I added it here.  I'm not sure if this is the right way to do it -- I don't even know if the `algorithms.component.rst` is supposed to be hand-modified or if it's supposed to be generated automatically from the modules in the `algorithms` directory.  This PR is also not tested because I'm not sure how to do that.