docs: Clarify that useQuery results are immutable

[ericyd]

This PR probably feels unnecessary to the maintainers, but it caused some confusion for me and seems to come up from time to time in various discussions and issue reports, e.g.

• TypeError: Cannot assign to read only property '0' of object '[object Array]' #7361
• [vue-query]An error occurred when attempting to modify the data returned by useQuery #4750
• Return a copy of data from useQuery #1304

In React, immutability is a core design choice baked into most operations. For React, immutable data should be intuitive for most developers.

In contrast, Vue treats mutability as a feature, even highlighting deep reactivity in their documentation and providing recommendations on two-way bindings with v-model. For the average Vue developer, I would wager that the default assumption is that data is mutable.

useQuery properly annotates the data as readonly in TypeScript. However, the TypeScript compiler (at least as of 5.8.3) does not protect against all operations on readonly objects. For example, this is valid and compiles just fine:

const immutable: Readonly<Record<string, boolean[]>> = { list: [true] }
immutable.list.push(false)

(Of course, assignment is forbidden):

const immutable: Readonly<Record<string, boolean[]>> = { list: [true] }
immutable.list = [false]
^^^^^^^^^^^^^^
Index signature in type 'Readonly<Record<string, boolean[]>>' only permits reading.

In addition Vue provides a warning when immutable object mutations are attempted. However, warnings can be easy to overlook and this can lead to bugs.

Therefore, I believe being explicit about immutability is a useful contribution for the Vue framework docs.