Move command description to docstring

As suggested in code review, this adds an actual docstring to the
script, and reuses the docstring as the ArgumentParser description.

bin/navsnmp

@@ -14,6 +14,10 @@
 # details.  You should have received a copy of the GNU General Public License
 # along with NAV. If not, see <http://www.gnu.org/licenses/>.
 #
+"""Outputs NAV's SNMP configuration for input devices as NET-SNMP compatible command
+line arguments.
+
+"""
 from __future__ import print_function
 import argparse
 import sys
@@ -41,8 +45,7 @@ def main():
 def parse_args():
     """Builds an ArgumentParser and returns parsed program arguments"""
     parser = argparse.ArgumentParser(
-        description="Outputs NAV's SNMP configuration of input devices as NET-SNMP "
-        "compatible command line arguments",
+        description=__doc__.replace("\n", " ").strip(),
         epilog="Example usage:\nsnmpwalk $(%(prog)s example-sw.example.org) "
         "SNMPv2-MIB::system ",
     )