Add XML documentation for Android.Graphics.Pdf.Content

The `Android.Graphics.Pdf.Content` namespace contains **124** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Graphics.Pdf.Content/**' --glob 'ns-Android.Graphics.Pdf.Content.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **114** to **124** (increased by **10**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to inspect the 124 matching To be added placeholders under Android.Graphics.Pdf.Content, excluding docs\xml\index.xml. Read the surrounding source XML documentation and the referenced API context before writing accurate entries. Done means the placeholders in this namespace are replaced and the scan no longer reports them.

索引モデルが issue の本文から書いたものです。