Minor improvements to FNSS troubleshooting script

This commit makes the output of the FNSS troubleshooting script more
concise and makes it print usage information if --help or -h
arguments are passed.
Add scripts documentation to core library API reference

This commit adds documentation of the scripts provided with the core
library, i.e. *fnss-troubleshoot* and *mn-fnss*. Before this commit,
they were not mentioned in the documentation and could be discovered
only by browsing the source code.