Update documentation around indexing specific cores individually

[justinthiele]

I struggled to figure out how to build/delete/check the indexes of core types individually. Now that I have it figured out, I updated the documentation to be a bit clearer on this type of usage.

[justinthiele]

Hi @yvanzo, thanks for reviewing my PR right away and providing thorough explanations.

The existing documentation makes sense to me now. I didn't pick up on the technical nuance until you explained it. Maybe it'd be helpful to add example usage to each? I'll do that in another commit and see what you think.

[justinthiele]

@yvanzo have a look at the new commit I just pushed.

[justinthiele]

@yvanzo Good catch, that duplicate --entity-type artist was a typo. Thanks for merging and for all of your work on this repo 👍