docs: update environment variable descriptions for clarity (#44)

Env vars are available to all platforms,
Singular form requires manual `docker-compose.yml` export

Author: kallebysantos

docs/environment-variables.md

@@ -5,11 +5,11 @@ On Supabase Platform and Local Development (CLI), all variables are auto-provisi
 | Variable                    | Format                             | Description                           | Available in                      |
 | --------------------------- | ---------------------------------- | ------------------------------------- | --------------------------------- |
 | `SUPABASE_URL`              | `https://<ref>.supabase.co`        | Your Supabase project URL             | All                               |
-| `SUPABASE_PUBLISHABLE_KEYS` | `{"default":"sb_publishable_..."}` | Named publishable keys as JSON object | Platform, Local Development (CLI) |
-| `SUPABASE_SECRET_KEYS`      | `{"default":"sb_secret_..."}`      | Named secret keys as JSON object      | Platform, Local Development (CLI) |
-| `SUPABASE_JWKS`             | `{"keys":[...]}` or `[...]`        | JSON Web Key Set for JWT verification | Platform, Local Development (CLI) |
-| `SUPABASE_PUBLISHABLE_KEY`  | `sb_publishable_...`               | Single publishable key (fallback)     | Self-hosted                       |
-| `SUPABASE_SECRET_KEY`       | `sb_secret_...`                    | Single secret key (fallback)          | Self-hosted                       |
+| `SUPABASE_PUBLISHABLE_KEYS` | `{"default":"sb_publishable_..."}` | Named publishable keys as JSON object | All                               |
+| `SUPABASE_SECRET_KEYS`      | `{"default":"sb_secret_..."}`      | Named secret keys as JSON object      | All                               |
+| `SUPABASE_JWKS`             | `{"keys":[...]}` or `[...]`        | JSON Web Key Set for JWT verification | All                               |
+| `SUPABASE_PUBLISHABLE_KEY`  | `sb_publishable_...`               | Single publishable key (fallback)     | Self-hosted, if manually exported |
+| `SUPABASE_SECRET_KEY`       | `sb_secret_...`                    | Single secret key (fallback)          | Self-hosted, if manually exported |
 
 ## Non-Supabase environments (Node.js, Bun, Cloudflare, self-hosted)