Fix port leaking after controlling_process(Port, self())

Add case to handle the situation when someone call
{gen_tcp,gen_udp}:controlling_process(Port, self()). Also improve spec
and doc from gen_udp and gen_sctp for controlling_process/2.

To reproduce the issue, open an UDP port:

4> {ok,Port} = gen_udp:open(9000, [binary]).
{ok,#Port<0.587>}
5> gen_udp:controlling_process(Port, self()).
ok

Simulate error:

6> 1=2.
** exception error: no match of right hand side value 2

Here is the leak:

7> inet:i().
Port Module   Recv Sent Owner    Local Address Foreign Address State
Type
581  inet_udp 0    0    <0.31.0> *:cslistener  *:*             BOUND
DGRAM
ok

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/kernel/doc/src/gen_udp.xml b/lib/kernel/doc/src/gen_udp.xml
index daa9b7d887..9882d3fab1 100644
--- a/lib/kernel/doc/src/gen_udp.xml
+++ b/lib/kernel/doc/src/gen_udp.xml
@@ -136,7 +136,9 @@
       <desc>
         <p>Assigns a new controlling process <c><anno>Pid</anno></c> to
           <c><anno>Socket</anno></c>. The controlling process is the process which
-          receives messages from the socket.</p>
+          receives messages from the socket. If called by any other
+          process than the current controlling process,
+          <c>{error, not_owner}</c> is returned.</p>
       </desc>
     </func>
     <func>