clarify requirements for a link local route

[kjetilho]

Description

This gives a better hint on how to add a link local route. Ie., if the user tries to add a local route (without a gateway (via)), they will get an error message that a global route needs via, and hopefully understand they should use scope: local. Also, the docs spell this out.

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[schopin-pro]

LGTM, I recall having a discussion on IRC on this particular point last week, which indicates a clear need ;-)

[codecov-commenter]

Codecov Report

Merging #226 (fe54301) into main (c6613b5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #226   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          56       56           
  Lines        9343     9343           
=======================================
  Hits         9252     9252           
  Misses         91       91           

Impacted Files	Coverage Δ
src/parse.c	100.00% <100.00%> (ø)

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[slyon]

Thank you for clarifying this bit of our documentation! LGTM as well!