expanded comment to explain method.

src/app/data_harvester/memory/sysinfo.rs

@@ -37,7 +37,8 @@ pub(crate) fn get_swap_usage(sys: &System) -> Option<MemHarvest> {
     })
 }
 
-/// Returns cache usage.
+/// Returns cache usage. sysinfo has no way to do this directly but it should equal the difference
+/// between the available and free memory. On windows, this will always be 0. TODO: hide on windows?
 pub(crate) fn get_cache_usage(sys: &System) -> Option<MemHarvest> {
     let mem_used_in_kib = (sys.available_memory() - sys.free_memory()) / 1024;
     let mem_total_in_kib = sys.total_memory() / 1024;