doc, tls: mark parseCertString() as deprecated

`tls.parseCertString()` was made public by mistack. So mark it as
deprecated.

PR-URL: #14245
Refs: #14193
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/deprecations.md

@@ -660,6 +660,28 @@ Type: Runtime
 
 `REPLServer.parseREPLKeyword()` was removed from userland visibility.
 
+<a id="DEP00XX"></a>
+### DEP00XX: tls.parseCertString()
+
+Type: Documentation-only
+
+`tls.parseCertString()` is a trivial parsing helper that was made public by
+mistake. This function can usually be replaced with:
+
+```js
+const querystring = require('querystring');
+querystring.parse(str, '\n', '=');
+```
+
+*Note*: This function is not completely equivalent to `querystring.parse()`. One
+difference is that `querystring.parse()` does url encoding:
+
+```sh
+> querystring.parse('%E5%A5%BD=1', '\n', '=');
+{ '好': '1' }
+> tls.parseCertString('%E5%A5%BD=1');
+{ '%E5%A5%BD': '1' }
+```
 
 [`Buffer.allocUnsafeSlow(size)`]: buffer.html#buffer_class_method_buffer_allocunsafeslow_size
 [`Buffer.from(array)`]: buffer.html#buffer_class_method_buffer_from_array