docs/pki: add remove_roots_from_chain option to /pki/issue (#21161)

Adds the `remove_roots_from_chain` option to the docs for
`/pki/issue`, based on addNonCACommonFields().

website/content/api-docs/secret/pki.mdx

@@ -637,6 +637,10 @@ It is suggested to limit access to the path-overridden issue endpoint (on
   `YYYY-MM-ddTHH:MM:SSZ`. Supports the Y10K end date for IEEE 802.1AR-2018
   standard devices, `9999-12-31T23:59:59Z`.
 
+- `remove_roots_from_chain` `(bool: false)` - If true, the returned `ca_chain`
+  field will not include any self-signed CA certificates. Useful if end-users
+  already have the root CA in their trust store.
+
 - `user_ids` `(string: "")` - Specifies the comma-separated list of requested
   User ID (OID 0.9.2342.19200300.100.1.1) Subject values to be placed on the
   signed certificate. This field is validated against `allowed_user_ids` on