[ZainnQureshii]

What does this PR do?

The devices section of the Fabric arguments page (docs/source-fabric/api/fabric_args.rst) shows two examples as the default, and neither works:

• Fabric(devices=None) with the comment "default used by Fabric, i.e., use the CPU". The default is devices="auto" (src/lightning/fabric/fabric.py:139), and None raises a TypeError on CPU and on GPU/MPS: _parse_cpu_cores rejects it on CPU (src/lightning/fabric/accelerators/cpu.py:98-99) and _check_data_type rejects it for GPU/MPS device IDs (src/lightning/fabric/utilities/device_parser.py:197-199).
• Fabric(devices=0) with the comment "equivalent". _check_device_config_and_set_final_flags rejects 0 with a ValueError (src/lightning/fabric/connector.py:305-314).

This PR replaces them with Fabric(devices="auto") for the default and Fabric(devices=1, accelerator="cpu") for an explicit CPU run, and adds "auto" to the str bullet above the block. The comment wording follows the Trainer reference (docs/source-pytorch/common/trainer.rst:429-433).

Ran each call against this branch's src/ (dependencies from lightning 2.6.6) on an Apple Silicon Mac (the auto accelerator picks MPS there), plus the old calls with accelerator="cpu":

{'devices': None} TypeError Device IDs (GPU/TPU) must be an int, a string, a sequence of ints, but you passed None
{'devices': 0} ValueError `Fabric(devices=0)` value is not a valid input using auto accelerator.
{'devices': None, 'accelerator': 'cpu'} TypeError `devices` selected with `CPUAccelerator` should be an int > 0.
{'devices': 0, 'accelerator': 'cpu'} ValueError `Fabric(devices=0)` value is not a valid input using cpu accelerator.
{'devices': 'auto'} OK mps:0
{'devices': 1, 'accelerator': 'cpu'} OK cpu

The docs build was not run locally.

Docs only, so no linked issue, tests or CHANGELOG entry.

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