Tooltips: Prometheus dashboards: Head Block: Update graph panel description

It is a simple task for PMM Contributions project.

Many widgets do not have tooltips.
I'll add a tooltip for one of the widgets to make an instruction for the community.

Prometheus -> Head Block:

The Head Block is kept in memory. Older blocks are accessed through mmap().
This eliminates the need to configure cache separately, but also means you need to allocate
plenty of memory for OS Cache if you want to query data older than fits in the Head Block.