http_async_client: add remarks section with a note about libcurl leak in centos 6

- based on GH #853

modules/http_async_client/doc/http_async_client_admin.xml

@@ -659,5 +659,18 @@ http_async_query("https://example.com/test.php", "HTTP_REPLY");
 	</section>
 
 </section>
+
+	<section id="http_async_client.s.remarks">
+		<title>Remarks</title>
+		<para>
+			Note: libcurl leak in CentOS 6 - this module uses libcurl library
+			and in case if you are using CentOS 6, be aware that standard
+			libcurl-7.19.7-52 has a memory leak. To fix this memory, install
+			libcurl from city-fan repository. More details at:
+			<ulink url="https://www.digitalocean.com/community/questions/how-to-upgrade-curl-in-centos6">
+				https://www.digitalocean.com/community/questions/how-to-upgrade-curl-in-centos6</ulink>
+		</para>
+	</section>
+
 </chapter>