Docs: Update network scanning documentation

[cakekoa]

What does this PR do?

Fixes part of #2299.

Updates the documentation for the "Scan Agent's networks" option.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

Base: 60.40% // Head: 60.56% // Increases project coverage by +0.16% 🎉

Coverage data is based on head (e63409d) compared to base (8b8ef79).
Patch coverage: 80.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2357      +/-   ##
===========================================
+ Coverage    60.40%   60.56%   +0.16%     
===========================================
  Files          539      540       +1     
  Lines        14075    14090      +15     
===========================================
+ Hits          8502     8534      +32     
+ Misses        5573     5556      -17     

Impacted Files	Coverage Δ
...agent_configuration/default_agent_configuration.py	100.00% <ø> (ø)
monkey/infection_monkey/master/propagator.py	99.00% <ø> (+<0.01%)	⬆️
...nkey/monkey_island/cc/services/reporting/report.py	25.64% <0.00%> (ø)
...on/agent_configuration/agent_sub_configurations.py	100.00% <100.00%> (ø)
...n_monkey/network_scanning/scan_target_generator.py	100.00% <100.00%> (ø)
monkey/tests/common/example_agent_configuration.py	100.00% <100.00%> (ø)
...ection_monkey/post_breach/custom_pba/custom_pba.py	62.29% <0.00%> (-1.04%)	⬇️
monkey/infection_monkey/monkey.py	0.00% <0.00%> (ø)
monkey/common/agent_events/__init__.py	100.00% <0.00%> (ø)
monkey/monkey_island/cc/models/agent.py	100.00% <0.00%> (ø)
... and 21 more

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