Simplify content warning dialog text

[surfdude29]

After considering feedback on Crowdin about the second sentence of the current explanatory text displayed in the Add a content warning dialog being confusing:

This may be overstepping a bit, but I believe if you want people to use this function, this is a confusing way to put it. ("If none are selected, this post is suitable for all audiences." is a technical description for how to interpret absence of labels after the fact, but runs counter-causality as guidance while authoring a post!)

I would instead write "If the media is suitable for all audiences, select none." or something along those lines.

I think it might be clearer to simplify the whole string and turn it into a polite request to add a content warning if it's applicable.

So this PR proposes changing the text from:

Choose self-labels that are applicable for the media you are posting. If none are selected, this post is suitable for all audiences.

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Please add any content warning self-labels that are applicable for the media you are posting.

cc: @Tamschi