Improve documentation for React.Children.only

[bvaughn]

This issue was originally reported by @jimfb via facebook/react/issues/4424

Please refer to the original issue for the related discussion thread.

My understanding of the outcome is that the docs and the implementation are slightly misaligned. The docs currently say:

Returns the only child in children. Throws otherwise.

This is somewhat ambiguous though with regard to how it handles an array with only 1 child element. The actual implementation uses a helper method to verify that the child is an object (eg not in an array).

Let's add some clarification to the docs about exactly what this method does.

[renatoselenica]

Can I take this one? If no one else is working on it right now.

[bvaughn]

This issue is all yours! 😄

I've added an "in-progress" label so that others will know not to start work on the issue. If you change your mind about the issue, no worries! Just let me know so that I can remove the label and free it up for someone else to claim.

Cheers!

[renatoselenica]

#118 I just submitted a pull request on it.
P.s. sorry I dont know how to do an actual pull request reference like the one above. Still new at this

[bvaughn]

No problem @renatoselenica.

For future reference, if your pull request description says something like "Fixes #87" then GitHub will automatically link the 2 and close the issue when the PR is merged.