doc: add multi buffer matching documentation v3

[jmtaylor90]

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

• I have read the contributing guide lines at https://suricata.readthedocs.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/6032

Describe changes:

• Add documentation describing multi buffer matching behavior
• Add links to relevant keyword documentation for multi buffer matching
• Address comments/feedback from doc: add multi buffer matching documentation v2 #9161

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 #9174 (4893467) into master (9b09b29) will increase coverage by 0.00%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9174   +/-   ##
=======================================
  Coverage   82.34%   82.35%           
=======================================
  Files         968      968           
  Lines      273546   273546           
=======================================
+ Hits       225247   225270   +23     
+ Misses      48299    48276   -23     

Flag	Coverage Δ
fuzzcorpus	64.60% <ø> (+<0.01%)	⬆️
suricata-verify	60.73% <ø> (-0.02%)	⬇️
unittests	62.91% <ø> (+<0.01%)	⬆️

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

[jmtaylor90]

continued in #9180