docs: planners return None when there is no path, say so in :return:

[petercorke]

Stacked on #26 (test/self-loops-parallel-edges): the base of this PR is that branch.

Summary

Documentation only. The three planners (path_BFS, path_UCS, path_Astar) already return None when there is no path, but only the prose said so; :return: and :rtype: described just the success case, which invites an unpack error on None.

• :return: / :rtype: now say "or None if there is no path".
• The prose says to check the result before unpacking it.

No code changes.

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[codacy-production]

Up to standards ✅

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.37%. Comparing base (5beab4e) to head (dda092c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #33   +/-   ##
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  Coverage   87.37%   87.37%           
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  Files           2        2           
  Lines         800      800           
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  Hits          699      699           
  Misses        101      101           

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