docs: Add float16 precision warning for optimizer eps and small values

[bhimrazy]

What does this PR do?

Adds a warning to the 16-bit precision documentation about float16's inability to represent values smaller than ~6e-5, which can cause NaN during training.

Fixes #20619

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📚 Documentation preview 📚: https://pytorch-lightning--21295.org.readthedocs.build/en/21295/