Recommend flutter config --help for an explanation of each feature flag

[loic-sharma]

Bubble up that flutter config --help explains what each flag controls.

cc @vashworth @matanlurey

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Visit the preview URL for this PR (updated for commit 95da1c9):

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[matanlurey]

My concern with this change is that it is likely to go out of date with flutter config --help. I think it could be valuable to list flags we expect to stick around for long periods of time, or perhaps to talk about we have a few different categories of flags as you listed, but otherwise I'd say away with trying to duplicate the command line documentation.

[loic-sharma]

@matanlurey and I discussed offline. Instead of explaining each flag here, I switched to recommending flutter config --help to get an explanation of flags.

[sfshaza2]

lgtm. Thanks, @loic-sharma, for adding this info. And thanks, @matanlurey, for making the docs more robust over time.