Fix supported schemes in docs

xml/System.Net.Http/HttpClient.xml

@@ -362,7 +362,15 @@ The environment variables used for `DefaultProxy` initialization on Windows and
 
 On systems where environment variables are case-sensitive, the variable names may be all lowercase or all uppercase.  The lowercase names are checked first.
 
-The proxy server may be a hostname or IP address, optionally followed by a colon and port number, or it may be an http URL, optionally including a username and password for proxy authentication. The URL must be start with `http`, not `https`, and cannot include any text after the hostname, IP, or port.
+The proxy server may be a hostname or IP address, optionally followed by a colon and port number, or it may be an http, https, socks4, socks4a or socks5 URL, optionally including a username and password for proxy authentication. The URL must be start with `http`, `https`, `socks4`, `socks4a` or `socks5`, and cannot include any text after the hostname, IP, or port.
+The URL can optionally include a username and password for proxy authentication for the schemes `http`, `https` and `socks5`. For the schemes `socks4` and `socks4a` only an userid is supported.
+
+* **`http`**: `http://[username:password@]hostname[:port]`
+* **`https`**: `https://[username:password@]hostname[:port]`
+* **`socks4`**: `socks4://[userid@]hostname[:port]`
+* **`socks4a`**: `socks4a://[userid@]hostname[:port]`
+* **`socks5`**: `socks5://[username:password@]hostname[:port]`
+
  ]]></format>
         </remarks>
         <exception cref="T:System.ArgumentNullException">The value passed cannot be <see langword="null" />.</exception>