/usr/lib/erlang/lib/diameter-1.5/doc is in erlang-doc 1:16.b.3-dfsg-1ubuntu2.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/erlang/lib/diameter-1.5/../../../../share/doc/erlang-doc/lib/diameter-1.5/doc