doc: clarify 'filename' argument in 'loadwallet' RPC #31031

Update the 'loadwallet' RPC help text to specify that the 'filename' should be provided as a path relative to the wallets directory. Add examples showing how to load a wallet from a different directory.

<!-- *** Please remove the following help text before submitting: *** Pull requests without a rationale and clear improvement may be closed immediately. GUI-related pull requests should be opened against https://github.com/bitcoin-core/gui first. See CONTRIBUTING.md -->

<!-- Please provide clear motivation for your patch and explain how it improves Bitcoin Core user experience or Bitcoin Core developer experience significantly: * Any test improvements or new tests that improve coverage are always welcome. * All other changes should have accompanying unit tests (see `src/test/`) or functional tests (see `test/`). Contributors should note which tests cover modified code. If no tests exist for a region of modified code, new tests should accompany the change. * Bug fixes are most welcome when they come with steps to reproduce or an explanation of the potential issue as well as reasoning for the way the bug was fixed. * Features are welcome, but might be rejected due to design or scope issues. If a feature is based on a lot of dependencies, contributors should first consider building the system outside of Bitcoin Core, if possible. * Refactoring changes are only accepted if they are required for a feature or bug fix or otherwise improve developer experience significantly. For example, most "code style" refactoring changes require a thorough explanation why they are useful, what downsides they have and why they *significantly* improve developer experience or avoid serious programming bugs. Note that code style is often a subjective matter. Unless they are explicitly mentioned to be preferred in the [developer notes](/doc/developer-notes.md), stylistic code changes are usually rejected. -->

<!-- Bitcoin Core has a thorough review process and even the most trivial change needs to pass a lot of eyes and requires non-zero or even substantial time effort to review. There is a huge lack of active reviewers on the project, so patches often sit for a long time. -->

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage

For detailed information about the code coverage, see the test coverage report.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process. A summary of reviews will appear here.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #30302 (doc: clarify loadwallet path loading for wallets by am-sq)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Duplicate of #30302