doc, tls: mark parseCertString() as deprecated

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

Refs: nodejs#14193
Refs: nodejs@af80e7b#diff-cc32376ce1eaf679ec2298cd483f15c7R188

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 URLDecoding:
+
+```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