Fix exceptions thrown by `setup.py build_man`

While doing some doc updates I needed a way to test them - to build
the documentation and inspect the output. I ran into an issue:
running python setup.py build_man was throwing exceptions:

1. The import-tar parser had a None description causing:

    File "/home/user/borg/setup_docs.py", line 451, in write_heading
      write(char * len(header))
    TypeError: object of type 'NoneType' has no len()

2. There was no docs/usage/import-tar.rst causing an exception too

1 files changed, 1 insertions, 0 deletions

diff --git a/docs/usage/import-tar.rst b/docs/usage/import-tar.rst
new file mode 100644
index 000000000..4939cbc29
--- /dev/null
+++ b/docs/usage/import-tar.rst
@@ -0,0 +1 @@
+.. include:: import-tar.rst.inc