Poorly/undocumented behaviors around subscription handling

We have poorly / undocumented behaviors on how subscription is handled when `--subscription` isn't passed and when certain `--subscription` values are passed.

From searching, we don't document that the logged into default subscription will be used before `AZURE_SUBSCRIPTION_ID` if `--subscription` isn't passed. We should clearly document all ways subscription is determined and the order they take precedent.

We also don't document special `--subscription` values. We have a concept of placeholder values where if the `--subscription` value contains either `subscription` or `default` in it we try to fallback to environment setup (the default logged in subscription or the `AZURE_SUBSCRIPTION_ID` environment variable). For this one we either should document this behavior or remove it, as subscription names having `subscription` or `default` in them is valid and falling back to the environment could be handled by not passing `--subscription` (and should be easier to understand).

No file or test is named. Start by locating the CLI entry point and subscription-resolution logic for --subscription, then trace the precedence between the logged-in default subscription and AZURE_SUBSCRIPTION_ID. Done means the precedence and special values are documented clearly, or the placeholder behavior is removed, with the chosen behavior verified.

Written by the indexing model from the issue text.