docs: Clarify the impact of updating contextual store values in SSR (#…

…10086)


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Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

documentation/docs/20-core-concepts/50-state-management.md

@@ -113,6 +113,8 @@ You might wonder how we're able to use `$page.data` and other [app stores](modul
 <p>Welcome {$user.name}</p>
 ```
 
+Updating the context-based store value in deeper-level pages or components will not affect the value in the parent component when the page is rendered via SSR: The parent component has already been rendered by the time the store value is updated. To avoid values 'flashing' during state updates during hydration, it is generally recommended to pass state down into components rather than up.
+
 If you're not using SSR (and can guarantee that you won't need to use SSR in future) then you can safely keep state in a shared module, without using the context API.
 
 ## Component state is preserved
@@ -167,4 +169,4 @@ If you have state that should survive a reload and/or affect SSR, such as filter
 
 ## Storing ephemeral state in snapshots
 
-Some UI state, such as 'is the accordion open?', is disposable — if the user navigates away or refreshes the page, it doesn't matter if the state is lost. In some cases, you _do_ want the data to persist if the user navigates to a different page and comes back, but storing the state in the URL or in a database would be overkill. For this, SvelteKit provides [snapshots](snapshots), which let you associate component state with a history entry.
+Some UI state, such as 'is the accordion open?', is disposable — if the user navigates away or refreshes the page, it doesn't matter if the state is lost. In some cases, you _do_ want the data to persist if the user navigates to a different page and comes back, but storing the state in the URL or in a database would be overkill. For this, SvelteKit provides [snapshots](snapshots), which let you associate component state with a history entry.