docs: update --subscription global option to document default resolution

The --subscription parameter description only mentioned AZURE_SUBSCRIPTION_ID
as the fallback. Since #1974 merged, the resolution order is:

1. Explicit --subscription value
2. Azure CLI profile (~/.azure/azureProfile.json, set via 'az account set')
3. AZURE_SUBSCRIPTION_ID environment variable

Update the Global Options table to reflect this and accept display names.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: diberry

servers/Azure.Mcp.Server/docs/azmcp-commands.md

@@ -9,7 +9,7 @@ The following options are available for all commands:
 
 | Option | Required | Default | Description |
 |-----------|----------|---------|-------------|
-| `--subscription` | No | Environment variable `AZURE_SUBSCRIPTION_ID` | Azure subscription ID for target resources |
+| `--subscription` | No | Azure CLI profile (`az account set`), then `AZURE_SUBSCRIPTION_ID` env var | Specifies the Azure subscription to use. Accepts either a subscription ID (GUID) or display name. If not specified, the default subscription is resolved from the Azure CLI profile (`~/.azure/azureProfile.json`, configured via `az account set`) or, if not set there, from the `AZURE_SUBSCRIPTION_ID` environment variable. |
 | `--tenant-id` | No | - | Azure tenant ID for authentication |
 | `--auth-method` | No | 'credential' | Authentication method ('credential', 'key', 'connectionString') |
 | `--retry-max-retries` | No | 3 | Maximum retry attempts for failed operations |