Reference R2 credentials over AWS in mounting docs (#29046)

Co-authored-by: scuffi <aferguson@cloudflare.com>

Author: scuffi

src/content/docs/sandbox/guides/mount-buckets.mdx

@@ -60,10 +60,14 @@ Mounted buckets are visible across all sessions since they share the filesystem.
 Set credentials as Worker secrets and the SDK automatically detects them:
 
 ```sh
-npx wrangler secret put AWS_ACCESS_KEY_ID
-npx wrangler secret put AWS_SECRET_ACCESS_KEY
+npx wrangler secret put R2_ACCESS_KEY_ID
+npx wrangler secret put R2_SECRET_ACCESS_KEY
 ```
 
+:::note[R2 credentials]
+We also automatically detect `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` for compatibility with other S3-compatible providers.
+:::
+
 <TypeScriptExample>
 ```typescript
 // Credentials automatically detected from environment
@@ -307,6 +311,13 @@ For provider-specific configuration, see the [s3fs-fuse wiki](https://github.com
 
 **Solution**: Set credentials as Worker secrets:
 
+```sh
+npx wrangler secret put R2_ACCESS_KEY_ID
+npx wrangler secret put R2_SECRET_ACCESS_KEY
+```
+
+or
+
 ```sh
 npx wrangler secret put AWS_ACCESS_KEY_ID
 npx wrangler secret put AWS_SECRET_ACCESS_KEY