Escaping of the closing sigil when multiline syntax is used

[fuelen]

Environment

• Elixir & Erlang/OTP versions (elixir --version): 1.12.3 & 24

Current behavior

It is not clear how to escape closing symbols when multiline syntax is used with ~S sigil.
\""" works fine without ~S sigil:

iex(1)> """
...(1)> \"""
...(1)> """ |> IO.inspect() |> IO.puts()
"\"\"\"\n"
"""

but not with ~S sigil:

iex(2)> ~S"""
...(2)> \"""
...(2)> """ |> IO.inspect() |> IO.puts()
"\\\"\"\"\n"
\"""

Expected behavior

escaping """ using single \ shouldn't add \ to the output