detect/analyzer: add more details for flow_age - v1

[AkakiAlice]

Ticket: #6312

Make sure these boxes are checked accordingly before submitting your Pull Request -- thank you.

Contribution style:

• I have read the contributing guide lines at
  https://docs.suricata.io/en/latest/devguide/contributing/contribution-process.html

Our Contribution agreements:

• I have signed the Open Information Security Foundation contribution agreement at
  https://suricata.io/about/contribution-agreement/ (note: this is only required once)

Changes (if applicable):

• I have updated the User Guide (in doc/userguide/) to reflect the changes made
• I have updated the JSON schema (in etc/schema.json) to reflect all logging changes
  (including schema descriptions)
• I have created a ticket at
  https://redmine.openinfosecfoundation.org/projects/suricata/issues

Link to ticket: https://redmine.openinfosecfoundation.org/issues/6312

Describe changes:

• Add more details for the flow.age keyword

Provide values to any of the below to override the defaults.

• To use an LibHTP, Suricata-Verify or Suricata-Update pull request,
  link to the pull request in the respective _BRANCH variable.
• Leave unused overrides blank or remove.

SV_BRANCH=OISF/suricata-verify#2103

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.22%. Comparing base (30806ce) to head (4a1f9a3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12011   +/-   ##
=======================================
  Coverage   83.22%   83.22%           
=======================================
  Files         910      910           
  Lines      258136   258146   +10     
=======================================
+ Hits       214831   214849   +18     
+ Misses      43305    43297    -8     

Flag	Coverage Δ
fuzzcorpus	61.44% <0.00%> (+0.02%)	⬆️
livemode	19.39% <0.00%> (+<0.01%)	⬆️
pcap	44.44% <0.00%> (-0.02%)	⬇️
suricata-verify	62.74% <100.00%> (-0.01%)	⬇️
unittests	59.28% <0.00%> (-0.01%)	⬇️

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