docs: document clean commands for the tutorial

doc/tutorial/source/getting-started.rst

@@ -933,6 +933,44 @@ current libraries from the ``build`` directory, but some users may find
 it useful if their use case involves working with programs outside
 of the |ns3| directory.
 
+Clean
+=====
+
+Cleaning refers to the removal of artifacts (e.g. files) generated or edited
+by the build process.  There are different levels of cleaning possible:
+
++----------+-------------------+--------------------------------------------------------------------------------+
+| Scope    | Command           | Description                                                                    |
++==========+===================+================================================================================+
+| clean    | `./ns3 clean`     | Remove artifacts generated by the CMake configuration and the build            |
++----------+-------------------+--------------------------------------------------------------------------------+
+| distclean| `./ns3 distclean` | Remove artifacts from the configuration, build, documentation, test and Python |
++----------+-------------------+--------------------------------------------------------------------------------+
+| ccache   | `ccache -C`       | Remove all compiled artifacts from the ccache                                  |
++----------+-------------------+--------------------------------------------------------------------------------+
+
+`clean` can be used if the focus is on reconfiguring the way that ns-3 is
+presently being compiled.  `distclean` can be used if the focus is
+on restoring the ns-3 directory to an original state.
+
+The ccache lies outside of the ns-3 directory (typically in a hidden
+directory at `~/.cache/ccache`) and is shared across projects.
+Users should be aware that cleaning the ccache will cause cache misses
+on other build directories outside of the current working directory.
+Cleaning this cache periodically may be helpful to reclaim disk space.
+Cleaning the ccache is completely separate from cleaning any files
+within the ns-3 directory.
+
+Because clean operations involve removing files, the option conservatively
+refuses to remove files if one of the deleted files or directories lies
+outside of the current working directory.  Users may wish to precede the
+actual clean with a `--dry-run`, when in doubt about what the clean
+command will do, because a dry run will print the warning if one exists.
+For example::
+
+  ./ns3 clean --dry-run
+  ./ns3 clean
+
 One ns3
 =======