doc: clarify FileHandle error handling

[bobu-putheeckal]

Fixes: #51135

This clarifies that filehandle.close() and filehandle.stat() promises may reject when the underlying file system operation fails, and points readers to Common System Errors for code values.

Evidence checked:

• FileHandle.prototype.stat() calls fsCall(fstat, this, options) in lib/internal/fs/promises.js.
• fstat() uses binding.fstat(..., kUsePromises) and routes binding failures through handleErrorFromBinding.
• FileHandle::ClosePromise() in src/node_file.cc documents that the close promise rejects with a UVException if closing the fd fails.
• The linked issue includes a maintainer example showing filehandle.stat() rejecting with EBADF after the handle is closed.

Validation:

• git diff --check
• node tools/lint-md/lint-md.mjs doc/api/fs.md
• npx -q core-validate-commit --no-validate-metadata --tap HEAD