Update flags.adoc

Author: lihaoyi

docs/modules/ROOT/pages/cli/flags.adoc

@@ -146,6 +146,9 @@ mill -j4 __.compile
 
 You can also set Mill's parallelism to some multiple of the number of cores, e.g.
 `-j0.5C` to use half as many threads as cores, or `-j2C` to use twice as many threads as cores.
+These can be useful as xref:_repo_level_mill_options[] to configure an appropriate level
+of parallelism that scales based on the number of cores available (which might differ
+between e.g. developer laptops and CI machines)
 
 Every `mill` run generates an output file in `out/mill-chrome-profile.json` that can be
 loaded into the Chrome browser's `chrome://tracing` page for visualization.