Add X-Tenant-Id header info in the API key section (#694)

## Overview
Customers didn't know to include X-tenant-Id header. Doesn't hurt to
mention it in the section where we explain what each API key is.

## Type of change

**Type:**
Update existing documentation

To automatically close an issue when this PR is merged, use closing
keywords:
- Closes INF-868



<!-- For LangChain employees, if applicable: -->
- Linear issue:
https://linear.app/langchain/issue/INF-868/add-to-docs-that-you-need-to-add-x-tenant-id-in-the-header-with

## Checklist
<!-- Put an 'x' in all boxes that apply -->
- [x] I have read the [contributing guidelines](README.md)
- [x] I have tested my changes locally using `docs dev`
- [] All code examples have been tested and work correctly
- [] I have used **root relative** paths for internal links
- [ ] I have updated navigation in `src/docs.json` if needed
- I have gotten approval from the relevant reviewers
- (Internal team members only / optional) I have created a preview
deployment using the [Create Preview Branch
workflow](https://github.com/langchain-ai/docs/actions/workflows/create-preview-branch.yml)

## Additional notes
<!-- Any other information that would be helpful for reviewers -->

---------

Co-authored-by: Kathryn May <[email protected]>

Author: romain-priour-lc

src/langsmith/administration-overview.mdx

@@ -104,6 +104,13 @@ Service keys are similar to PATs, but are used to authenticate requests to the L
 
 Service keys are prefixed with `lsv2_sk_`
 
+<Warning>
+Use the `X-Tenant-Id` header to specify the target workspace.
+
+- **When using PATs**: If this header is omitted, requests will run against the default workspace associated with the key.
+- **When using organization-scoped service keys**: You must include the `X-Tenant-Id` header when accessing workspace-scoped resources. Without it, the request will fail with a `403 Forbidden` error.
+</Warning>
+
 <Note>
 To see how to create a service key or Personal Access Token, see the [setup guide](/langsmith/create-account-api-key)
 </Note>