Nexx360 BidAdapter: placement added (#6349)

* placement added

* Fix formatting of ID field requirement note

---------

Co-authored-by: Gabriel Chicoye <gabriel@macbookrogab24g.lan>

Author: gchicoye

dev-docs/bidders/nexx360.md

@@ -34,6 +34,10 @@ multiformat_supported: will-bid-on-any
 | `divId`       | optional | divId linked to adUnit     | `"div-1"`                                 | `string`  |
 | `adUnitName`  | optional | A code to identify adUnit  | `"header-ad"`                             | `string`  |
 | `adUnitPath`  | optional | A reference to adUnit Path | `"/12345/nexx360/Homepage/HP/Header-Ad"`  | `string`  |
+| `placement`   | optional | Nexx360 placement          | `"TEST_PLACEMENT"`                        | `string`  |
+
+You *must* only include one ID field - either `tagId` or `placement`, not both. If you have questions on which parameter to use, please reach out to your Account Manager.
+The `tagId` and `placement` are **mutually exclusive** but at least one is required. If you pass both, `tagId` takes precedence.
 
 ### Bidder Config