edoc generates several returns for functions having several specification clauses

[pguyot]

Describe the bug
When generating documentation about a function that has a spec with several clauses and a @returns tag, edoc duplicates the content of the returns tag.

To Reproduce

Create a file edoc_returns.erl

-module(edoc_returns).
-export([is_pid_and_not_list/1]).

%% @doc This returns true or false.
%% @end
%% @returns a boolean depending on the passed type.
-spec is_pid_and_not_list(Pid :: pid()) -> true;
                         (List :: list()) -> false.
is_pid_and_not_list(Pid) when is_pid(Pid) -> true;
is_pid_and_not_list(List) when is_list(List) -> false.

Generate documentation with:

edoc:files(["edoc_returns.erl"], []).

Check generated edoc_returns.html. It does contain:

<p>returns: a boolean depending on the passed type.</p>
<p>returns: a boolean depending on the passed type.</p>

Expected behavior
The returns paragraph should not be duplicated.

Affected versions
OTP22 ({edoc,"EDoc","0.11"}) is not affected. This was observed with:

• OTP23 ({edoc,"EDoc","0.12"})
• OTP24 ({edoc,"EDoc, the Erlang documentation engine","1.1"})
• OTP25 ({edoc,"EDoc, the Erlang documentation engine","1.2"})
• OTP26 ({edoc,"EDoc, the Erlang documentation engine","1.2"})
• and master (d3aa6c0)