doc: add tcp flags documentation v2

[jmtaylor90]

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

• I have read the contributing guide lines at https://docs.suricata.io/en/latest/devguide/codebase/contributing/contribution-process.html
• I have signed the Open Information Security Foundation contribution agreement at https://suricata.io/about/contribution-agreement/
• I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)

Link to redmine ticket:

https://redmine.openinfosecfoundation.org/issues/6349

Describe changes:

• Updated to include prefilter keyword example

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

To use a pull request use a branch name like pr/N where N is the
pull request number.

Alternatively, SV_BRANCH may also be a link to an
OISF/suricata-verify pull-request.

SV_REPO=
SV_BRANCH=
SU_REPO=
SU_BRANCH=
LIBHTP_REPO=
LIBHTP_BRANCH=

[codecov]

Codecov Report

Merging #9537 (281799d) into master (bb15a8f) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9537      +/-   ##
==========================================
- Coverage   82.19%   82.19%   -0.01%     
==========================================
  Files         968      968              
  Lines      274216   274216              
==========================================
- Hits       225405   225391      -14     
- Misses      48811    48825      +14     

Flag	Coverage Δ
fuzzcorpus	64.07% <ø> (+<0.01%)	⬆️
suricata-verify	60.90% <ø> (-0.03%)	⬇️
unittests	62.88% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[victorjulien]

Merged in #9539, thanks!