added documentation to cache memory data collection

src/app/data_harvester/memory/sysinfo.rs

@@ -38,7 +38,11 @@ pub(crate) fn get_swap_usage(sys: &System) -> Option<MemHarvest> {
 }
 
 /// 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.
+/// between the available and free memory. Free memory is defined as memory not containing any data,
+/// which means cache and buffer memory are not "free". Available memory is defined as memory able
+/// to be allocated by processes, which includes cache and buffer memory. On Windows, this will
+/// always be 0. For more information, see [docs](https://docs.rs/sysinfo/0.28.4/sysinfo/trait.SystemExt.html#tymethod.available_memory)
+/// and [memory explanation](https://askubuntu.com/questions/867068/what-is-available-memory-while-using-free-command)
 #[cfg(not(target_os = "windows"))]
 pub(crate) fn get_cache_usage(sys: &System) -> Option<MemHarvest> {
     let mem_used = sys.available_memory() - sys.free_memory();