chore: Adding flag & docs for jsxFragment

[rschristian]

https://github.com/developit/microbundle/issues/809#issuecomment-1025937798

I removed the options.jsx === 'React.createElement' thing as it's no longer relevant with jsxFragmentFactory (see https://github.com/developit/microbundle/pull/623#issuecomment-632442811) and we set default values in the CLI for both jsx and jsxFragment, so we don't really need extra fallback values.

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Latest commit: 8f6509f504711248b1857f0e5efe5d2399236e80

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| Name | Type | | ----------- | ----- | | microbundle | Patch |

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[ForsakenHarmony]

bors r+

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