Rename the update command

The `beet update` command name often (very often!) fools users into thinking that it performs a re-import or does what the mbsync plugin does. This is understandable: it sounds like it _updates_ the information from MusicBrainz.

I think it's time to rename the command. My current favorite name is `read`, since it's the opposite of the `write` command. Other suggestions have included `resync` and `refresh`.

We would of course keep `update` as an alias but leave that out of the documentation.

Start by tracing the `beet update` command and its user-facing documentation. Decide on the replacement name from the suggestions, preserve `update` as an alias, and remove the old name from documentation once the behavior and naming are agreed.

Written by the indexing model from the issue text.