/usr/lib/erlang/erts-7.3/doc is in erlang-doc 1:18.3-dfsg-1ubuntu3.1.
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It is a symlink to /usr/lib/erlang/erts-7.3/../../../share/doc/erlang-doc/erts-7.3/doc
/usr/lib/erlang/erts-7.3/../../../share/doc/erlang-doc/erts-7.3/doc