[Tile-serving] [osm2pgsql] Mention multi backend in usage (#261)

[Paul Norman]

Also explicitly mentions that the null backend doesn't disable the
middle, only the backend

Fixes #260

Clarifies #243
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/osm2pgsql/pull/261

-- Commit Summary --

  * Mention multi backend in usage

-- File Changes --

    M docs/usage.md (7)

-- Patch Links --

https://github.com/openstreetmap/osm2pgsql/pull/261.patch
https://github.com/openstreetmap/osm2pgsql/pull/261.diff

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