Help: Fix inaccuracies in INSTALL_REMOVE_ENVIRONMENT_RPATH docs

The previous docs assumed one had to set INSTALL_RPATH for
this target property to have an effect, but the actual condition is
only that the install rpath is different to the build rpath. Even if
the install rpath is empty, it is possible that CMake will rewrite
the rpath during install because the build rpath could be
non-empty.
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