Updated Setup/Test/Var/Yum files based on AA feedback

[Andriciuc]

setup.md:

Modified file to be postgresql.conf file
Modified last note to be clearer.

test.md:
UPdated function return for step 2 checking encrypted data

variables.md:

Added warning note for Enabling WAL encryption.

yum.md:

Updated Memory Lock description.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-17.5.2@8b1f1cf). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             release-17.5.2     #445   +/-   ##
=================================================
  Coverage                  ?   84.68%           
=================================================
  Files                     ?       21           
  Lines                     ?     2593           
  Branches                  ?      401           
=================================================
  Hits                      ?     2196           
  Misses                    ?      316           
  Partials                  ?       81           

Components	Coverage Δ
access	81.11% <0.00%> (?)
catalog	88.22% <0.00%> (?)
common	77.77% <0.00%> (?)
encryption	73.45% <0.00%> (?)
keyring	72.88% <0.00%> (?)
src	91.54% <0.00%> (?)
smgr	94.85% <0.00%> (?)
transam	∅ <0.00%> (?)

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