Update zsh auto-complete docs and narrow `_bash_complete()` scope

### Describe the feature

Related PR: https://github.com/aws/aws-cli/pull/2708

### Use Case

.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CLI version used

.

### Environment details (OS name and version, etc.)

.

Start by reviewing related PR #2708 and the zsh auto-complete documentation, then locate the `_bash_complete()` entry point in the CLI sources. Done means the documentation reflects the intended zsh behavior and `_bash_complete()` is limited to the required scope.

Written by the indexing model from the issue text.