docs: document that setDefaultOptions overwrites previous defaults

[fnune]

I noticed my default staleTime wasn't applying after I introduced a utility to namespace query keys. This was because I'd assumed setDefaultOptions would merge with previously-defined defaults, but it doesn't.

I fixed this in my project by merging with a default config object:

+ import { merge } from "lodash";

And then:

- queryClient.setDefaultOptions({
-     queries: {
-         queryKeyHashFn: myFunction,
-     },
- });
+ queryClient.setDefaultOptions(
+     merge<DefaultOptions, DefaultOptions>(
+         {
+             queries: {
+                 queryKeyHashFn: myFunction,
+             },
+         },
+         DEFAULT_QUERY_CLIENT_OPTIONS
+     )
+ );

I'm opening this PR to discuss mentioning this behavior in the docs. Additionally, setDefaultOptions could take a function where the existing options get passed.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/tannerlinsley/react-query/A6ZKKzv5w9FuoEvX7w3V3GKsD8o4
✅ Preview: https://react-query-git-fork-fnune-document-that-set-default-fb07f5.vercel.app

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 9a6faa4:

Sandbox	Source
tannerlinsley/react-query: basic	Configuration
tannerlinsley/react-query: basic-typescript	Configuration

[tannerlinsley]

🎉 This PR is included in version 3.16.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀