Add a tip about "use server" to action docs

[amirhhashemi]

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This PR adds a callout about "use server" to the Actions guide and the action API reference.

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[AlexErrant]

Sorry I'm a noob; what does this mean in the context of a Solid Start action?

use the "use server" directive to leverage server-side caching

Actions are for mutations, and I don't understand what server-side caching has to do with mutations. Perhaps a mutation could invalidate a cache key, but I don't think that's specific to server side caches? Also the invalidation would need to be explicit, so I don't think simply adding "use server" would do that.

[amirhhashemi]

@AlexErrant Thank you for reporting this. I think you are right. I'm rewriting this page and will this issue as well: #1179