Add new apt list rm command & apt clean description to Ubuntu article

This matches the previous commits' additions to the Pop!_OS article.

Author: jacobgkau

content/package-manager-ubuntu.md

@@ -24,6 +24,7 @@ If your system complains about a failed upgrade, package manager conflicts, brok
 
 ```bash
 sudo apt clean
+sudo rm -r /var/lib/apt/lists/*
 sudo apt update
 sudo dpkg --configure -a
 sudo apt install -f
@@ -33,7 +34,8 @@ sudo apt autoremove --purge
 
 ## What Do These Do?
 
-- `apt clean` - The `clean` command clears out the local repository of retrieved package files.
+- `apt clean` - The `clean` command clears out the local repository of retrieved package files (including older versions) from `/var/cache/apt/archives/`.
+- `sudo rm -r /var/lib/apt/lists/*` - This command clears all locally cached apt repository metadata (package lists and index files). This forces apt to fully re-fetch and rebuild its database of available packages on the next `apt update`.
 - `apt update` - the `update` option fetches indexes from all configured sources. These indexes are used by other apt options to determine which packages can be upgraded or installed.
 - `dpkg --configure -a` - The `--configure -a` command configures any unpacked but not yet configured packages.
 - `apt install -f` - The `-f` option attempts to correct broken dependencies