Maybe still not perfect, but there's at least a way to find out what
might make a callable "blockable" (and necessitate using `unblockable`
to change that), and how to bypass something that you don't want to be
blocked getting blocked.

We do still need proper documentation on how to use built-in commands
from `coretasks`, but I don't particularly want to link the migrated
wiki page at https://sopel.chat/usage/ignoring-people/ here.

That old stuff will continue getting folded into the Sphinx docs over
time. Arguably, describing the `.blocks` command in `configuration.rst`
is already enough to mark the old website page for deletion.

Eliminated the link (to a webpage we're probably going to delete) in
favor of directly specifying the available arguments, and added an
example of how to `del` a block. (The only examples before were `add`.)