docs: update callback return types to Promise<void> | void in mutation-related documentation

This commit refines the documentation for mutation-related callbacks (`onSuccess`, `onError`, `onSettled`, and `onMutate`), changing their return types from `Promise<unknown> | unknown` to `Promise<void> | void`.

Author: braden-w

docs/framework/react/plugins/persistQueryClient.md

@@ -214,12 +214,12 @@ ReactDOM.createRoot(rootElement).render(
 
 - `persistOptions: PersistQueryClientOptions`
   - all [options](#options) you can pass to [persistQueryClient](#persistqueryclient) minus the QueryClient itself
-- `onSuccess?: () => Promise<unknown> | unknown`
+- `onSuccess?: () => Promise<void> | void`
   - optional
   - will be called when the initial restore is finished
   - can be used to [resumePausedMutations](../../../../reference/QueryClient.md#queryclientresumepausedmutations)
   - if a Promise is returned, it will be awaited; restoring is seen as ongoing until then
-- `onError?: () => Promise<unknown> | unknown`
+- `onError?: () => Promise<void> | void`
   - optional
   - will be called when an error is thrown during restoration
   - if a Promise is returned, it will be awaited

docs/framework/react/reference/useMutation.md

@@ -68,15 +68,15 @@ mutate(variables, {
   - This function will fire before the mutation function is fired and is passed the same variables the mutation function would receive
   - Useful to perform optimistic updates to a resource in hopes that the mutation succeeds
   - The value returned from this function will be passed to both the `onError` and `onSettled` functions in the event of a mutation failure and can be useful for rolling back optimistic updates.
-- `onSuccess: (data: TData, variables: TVariables, context: TContext) => Promise<unknown> | unknown`
+- `onSuccess: (data: TData, variables: TVariables, context: TContext) => Promise<void> | void`
   - Optional
   - This function will fire when the mutation is successful and will be passed the mutation's result.
   - If a promise is returned, it will be awaited and resolved before proceeding
-- `onError: (err: TError, variables: TVariables, context?: TContext) => Promise<unknown> | unknown`
+- `onError: (err: TError, variables: TVariables, context?: TContext) => Promise<void> | void`
   - Optional
   - This function will fire if the mutation encounters an error and will be passed the error.
   - If a promise is returned, it will be awaited and resolved before proceeding
-- `onSettled: (data: TData, error: TError, variables: TVariables, context?: TContext) => Promise<unknown> | unknown`
+- `onSettled: (data: TData, error: TError, variables: TVariables, context?: TContext) => Promise<void> | void`
   - Optional
   - This function will fire when the mutation is either successfully fetched or encounters an error and be passed either the data or error
   - If a promise is returned, it will be awaited and resolved before proceeding

docs/reference/MutationCache.md

@@ -28,19 +28,19 @@ Its available methods are:
 
 **Options**
 
-- `onError?: (error: unknown, variables: unknown, context: unknown, mutation: Mutation) => Promise<unknown> | unknown`
+- `onError?: (error: unknown, variables: unknown, context: unknown, mutation: Mutation) => Promise<void> | void`
   - Optional
   - This function will be called if some mutation encounters an error.
   - If you return a Promise from it, it will be awaited
-- `onSuccess?: (data: unknown, variables: unknown, context: unknown, mutation: Mutation) => Promise<unknown> | unknown`
+- `onSuccess?: (data: unknown, variables: unknown, context: unknown, mutation: Mutation) => Promise<void> | void`
   - Optional
   - This function will be called if some mutation is successful.
   - If you return a Promise from it, it will be awaited
-- `onSettled?: (data: unknown | undefined, error: unknown | null, variables: unknown, context: unknown, mutation: Mutation) => Promise<unknown> | unknown`
+- `onSettled?: (data: unknown | undefined, error: unknown | null, variables: unknown, context: unknown, mutation: Mutation) => Promise<void> | void`
   - Optional
   - This function will be called if some mutation is settled (either successful or errored).
   - If you return a Promise from it, it will be awaited
-- `onMutate?: (variables: unknown, mutation: Mutation) => Promise<unknown> | unknown`
+- `onMutate?: (variables: unknown, mutation: Mutation) => Promise<void> | void`
   - Optional
   - This function will be called before some mutation executes.
   - If you return a Promise from it, it will be awaited