update documentation for new, neutered key-value backends

Backends are now only used to generate keys (and check them); they
are not arbitrary key-value stores for data, because it turned out such
a store is better modeled as a special remote. Updated docs to not
imply backends do more than they do now.

Sometimes I'm tempted to rename "backend" to "keytype" or something,
which would really be more clear. But it would be an annoying transition
for users, with annex.backends etc.

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