HPCC-9654 Add helpful text about treemaps

Add text describing how the treemaps are used in the timings page

Signed-off-by: Miguel Vazquez <miguel.vazquez@lexisnexis.com>
Merge pull request #4707 from miguelvazq/HPCC-9654

HPCC-9654 Add helpful text about treemaps

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>