Add fern api update prerequisite note and x-fern-sdk-namespace to exten…

[fern-api]

Two targeted improvements: (1) In the fern api update accordion on the General commands page, added a plain-prose note (inline, per AGENTS.md callout guidance) explaining that the local file referenced by openapi: in generators.yml must already exist before running the command — it overwrites in place, not creates — and that a Missing file: <filename> error is fixed by creating an empty placeholder with touch then re-running. (2) In the OpenAPI extensions overview table, added a new row for x-fern-sdk-namespace with the description "Namespace schemas to prevent name collisions when combining multiple API specs", placed after the x-fern-sdk-variables row, with no link since no dedicated page exists yet.

[github-actions]

🚫 [vale] <Microsoft.Contractions> _{reported by reviewdog 🐶}
Use 'doesn't' instead of 'does not'.

Suggested change

	The local file referenced by the `openapi:` path in `generators.yml` must already exist before running `fern api update`. The command overwrites the file in place — it does not create a new file. If the file is missing, the command outputs `Missing file: <filename>`. To fix this, create an empty placeholder at the expected path (e.g., `touch path/to/openapi.json`) and re-run `fern api update`.
	The local file referenced by the `openapi:` path in `generators.yml` must already exist before running `fern api update`. The command overwrites the file in place — it doesn't create a new file. If the file is missing, the command outputs `Missing file: <filename>`. To fix this, create an empty placeholder at the expected path (e.g., `touch path/to/openapi.json`) and re-run `fern api update`.

[github-actions]

🌿 Preview your docs: https://fern-preview-2026-09-02-fern-ai-2fbdc2-9ab99c48.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/cli-api-reference/cli-reference/general-commands