MINOR: muxes: Note that we can't usee a connection when added to the srv idle.

In the various muxes, add a comment documenting that once
srv_add_to_idle_list() got called, any thread may pick that conenction up,
so it is unsafe to access the mux context/the connection, the only thing we
can do is returning.

src/mux_fcgi.c

@@ -3558,6 +3558,10 @@ static void fcgi_detach(struct conn_stream *cs)
 					TRACE_DEVEL("outgoing connection killed", FCGI_EV_STRM_END|FCGI_EV_FCONN_ERR);
 					return;
 				}
+				/* At this point, the connection has been added to the
+				 * server idle list, so another thread may already have
+				 * hijacked it, so we can't do anything with it.
+				 */
 				TRACE_DEVEL("reusable idle connection", FCGI_EV_STRM_END, fconn->conn);
 				return;
 			}

src/mux_h1.c

@@ -2473,6 +2473,10 @@ static void h1_detach(struct conn_stream *cs)
 				TRACE_DEVEL("outgoing connection killed", H1_EV_STRM_END|H1_EV_H1C_END);
 				goto end;
 			}
+			/* At this point, the connection has been added to the
+			 * server idle list, so another thread may already have
+			 * hijacked it, so we can't do anything with it.
+			 */
 			return;
 		}
 	}

src/mux_h2.c

@@ -3959,6 +3959,10 @@ static void h2_detach(struct conn_stream *cs)
 						TRACE_DEVEL("leaving on error after killing outgoing connection", H2_EV_STRM_END|H2_EV_H2C_ERR);
 						return;
 					}
+					/* At this point, the connection has been added to the
+					 * server idle list, so another thread may already have
+					 * hijacked it, so we can't do anything with it.
+					 */
 					TRACE_DEVEL("reusable idle connection", H2_EV_STRM_END);
 					return;