docs: Adds note about default unit for fxFlex if none given

[peterramsing]

Maybe I'm missing something, but this might be helpful to specify.

[peterramsing]

I'm not entirely sure how my changes broke the build. I'll look at it later to see what's up.

[julianobrasil]

This info is already in the docs, but buried in the Responsive API wiki. I also think it's hard to find, dispite the fact that some examples kind of suggest it.

[peterramsing]

@julianobrasil, yup, that is pretty well doc'd in the Responsive API section. I'd almost want to bring that formatting to what I did if what I wrote should even go where I put it.

[peterramsing]

@ThomasBurleson, I suppose this PR might be the tip of a documentation iceberg for me. Is there any other documentation that I could help with? Are there any areas outside of #598 that I could help with?

[CaerusKaru]

Please rebase to resolve the test failure

[peterramsing]

@CaerusKaru Done.

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