docs(document): use a pdf file in the watermark example

Watermarking only applies to docx/pdf output; the other examples already
use docx, so show pdf here to make the supported formats clear.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: yoshiamakino

api-reference/openapi.json

@@ -1078,10 +1078,10 @@
                   }
                 },
                 "Watermark": {
-                  "summary": "Adding a \"Translated by DeepL\" watermark",
+                  "summary": "Adding a \"Translated by DeepL\" watermark (docx/pdf only)",
                   "value": {
                     "target_lang": "DE",
-                    "file": "@document.docx",
+                    "file": "@document.pdf",
                     "enable_watermark": true
                   }
                 },

api-reference/openapi.yaml

@@ -828,10 +828,10 @@ paths:
                   target_lang: DE
                   file: '@document.docx'
               Watermark:
-                summary: Adding a "Translated by DeepL" watermark
+                summary: Adding a "Translated by DeepL" watermark (docx/pdf only)
                 value:
                   target_lang: DE
-                  file: '@document.docx'
+                  file: '@document.pdf'
                   enable_watermark: true
               Glossary:
                 summary: Using a Glossary