feat: document -elem and -attr CSP directives

[ematipico]

Description (required)

This PR documents new APIs for CSP.

For Astro version: 7.1.0. See astro PR #17214.

[astrobot-houston]

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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[ArmandPhilippot]

Thanks, I left a few suggestions, mostly around `kind`: `'element'` because I think it might be confusing. My mind wants to correct this to kind: 'element'. So, it might be helpful to work around this.

Otherwise, this looks good to me. I was a bit scare by the big caution but I guess this is kinda expected for a tricky feature!

[ArmandPhilippot]

Ah, nice catch. I forgot about this pattern when I answered on Discord. I think it makes sense now that I have the content right in front of me!

[ArmandPhilippot]

Thank you for the update, LGTM!

[yanthomasdev]

I have a small nit but then LGTM