cody: document long context for BYOK only (#1349)

See
https://www.notion.so/sourcegraph/3581c98de162436096af66dd474c6ea3#263a8e1126588049bceadd7377e2d3b8
and
https://sourcegraph.slack.com/archives/C05GJPTSZCZ/p1757109608671939?thread_ts=1755766102.317339&cid=C05GJPTSZCZ

Amp-Thread-ID:
https://ampcode.com/threads/T-8e0b81d8-65e2-48dc-89c0-a18fa7c91fba

Co-authored-by: Amp <[email protected]>

Author: bobheadxi

docs/cody/enterprise/model-configuration.mdx

@@ -42,6 +42,8 @@ For most administrators, relying on these models alone ensures access to high-qu
 
 The use of these models is controlled through the `"modelConfiguration.sourcegraph"` field in the site config.
 
+<Callout type="note">Sourcegraph-provided models through Cody Gateway have context window limitations. To access models with extended context windows (such as Claude Sonnet 4's [1M token context](https://www.anthropic.com/news/1m-context)), configure BYOK (Bring Your Own Key) models using `providerOverrides` and suitable `contextWindow` configuration. See the [examples page](/cody/enterprise/model-config-examples) for BYOK configuration details.</Callout>
+
 ### Configure Sourcegraph-provided models
 
 The minimal configuration for Sourcegraph-provided models is: