docs(QTable): clarify virtual scroll usage with expanded rows and multiple QTr per data row (fix #18304)

[Aditya07771]

Closes #18304.

This PR adds two clarifying ::: tip blocks to the QTable virtual scrolling documentation.

It explains:

• When rendering more than one QTr for a single data row, each row should have a unique key
• Extra rows should usually use q-virtual-scroll--with-prev so VirtualScroll measures them together with the previous row
• Rows that should be excluded from size calculation should use q-virtual-scroll--skip
• This is especially important when combining virtual scroll with custom body slots, expanded rows, and sticky headers

The goal is to make the expected usage clearer and help users avoid incorrect scroll height, jumping scroll position, and expansion measurement issues.

[rstoenescu]

Thanks for contributing!