K8SPSMDB-1146 clarify incode the purpose of the databaseAdmin user

[gkech]

CHANGE DESCRIPTION

Problem:
Not a problem, clarify with a comment the purpose of the databaseAdmin user in our codebase and why it exists since it is not used by the operator.

Cause:
Short explanation of the root cause of the issue if applicable.

Solution:
Short explanation of the solution we are providing with this PR.

CHECKLIST

Jira

• Is the Jira ticket created and referenced properly?
• Does the Jira ticket have the proper statuses for documentation (Needs Doc) and QA (Needs QA)?
• Does the Jira ticket link to the proper milestone (Fix Version field)?

Tests

• Is an E2E test/test case added for the new feature/change?
• Are unit tests added where appropriate?
• Are OpenShift compare files changed for E2E tests (compare/*-oc.yml)?

Config/Logging/Testability

• Are all needed new/changed options added to default YAML files?
• Are all needed new/changed options added to the Helm Chart?
• Did we add proper logging messages for operator actions?
• Did we ensure compatibility with the previous version or cluster upgrade process?
• Does the change support oldest and newest supported MongoDB version?
• Does the change support oldest and newest supported Kubernetes version?

[egegunes]

while documenting this, let's add some descriptive comments to other users as well

[it-percona-cla]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[JNKPercona]

Test Name	Result	Time
arbiter	passed	00:11:30
balancer	passed	00:18:09
cross-site-sharded	passed	00:18:24
custom-replset-name	passed	00:10:23
custom-tls	passed	00:14:10
custom-users-roles	passed	00:11:01
custom-users-roles-sharded	passed	00:11:51
data-at-rest-encryption	passed	00:12:46
data-sharded	passed	00:22:28
demand-backup	passed	00:16:09
demand-backup-eks-credentials-irsa	passed	00:00:11
demand-backup-fs	passed	00:23:32
demand-backup-if-unhealthy	passed	00:08:26
demand-backup-incremental	passed	00:44:43
demand-backup-incremental-sharded	passed	00:57:08
demand-backup-physical-parallel	passed	00:08:14
demand-backup-physical-aws	passed	00:12:16
demand-backup-physical-azure	passed	00:12:06
demand-backup-physical-gcp-s3	passed	00:11:17
demand-backup-physical-gcp-native	passed	00:12:16
demand-backup-physical-minio	passed	00:20:18
demand-backup-physical-sharded-parallel	passed	00:10:24
demand-backup-physical-sharded-aws	passed	00:17:31
demand-backup-physical-sharded-azure	passed	00:18:02
demand-backup-physical-sharded-gcp-native	passed	00:18:10
demand-backup-physical-sharded-minio	passed	00:18:18
demand-backup-sharded	passed	00:25:45
expose-sharded	passed	00:33:40
finalizer	passed	00:10:09
ignore-labels-annotations	passed	00:08:19
init-deploy	passed	00:13:28
ldap	passed	00:09:01
ldap-tls	passed	00:13:14
limits	passed	00:06:18
liveness	passed	00:08:31
mongod-major-upgrade	passed	00:12:15
mongod-major-upgrade-sharded	passed	00:20:59
monitoring-2-0	passed	00:24:51
monitoring-pmm3	passed	00:27:15
multi-cluster-service	passed	00:13:12
multi-storage	passed	00:19:08
non-voting-and-hidden	passed	00:16:54
one-pod	passed	00:07:55
operator-self-healing-chaos	passed	00:13:11
pitr	passed	00:32:10
pitr-physical	passed	01:00:37
pitr-sharded	passed	00:22:43
pitr-to-new-cluster	passed	00:25:10
pitr-physical-backup-source	passed	00:53:59
preinit-updates	passed	00:05:28
pvc-resize	passed	00:13:53
recover-no-primary	passed	00:28:19
replset-overrides	passed	00:16:31
rs-shard-migration	passed	00:13:49
scaling	passed	00:11:37
scheduled-backup	passed	00:18:29
security-context	passed	00:07:24
self-healing-chaos	passed	00:15:58
service-per-pod	passed	00:19:10
serviceless-external-nodes	passed	00:07:47
smart-update	passed	00:08:30
split-horizon	passed	00:08:22
stable-resource-version	passed	00:05:05
storage	passed	00:08:15
tls-issue-cert-manager	passed	00:30:07
upgrade	passed	00:09:31
upgrade-consistency	passed	00:06:41
upgrade-consistency-sharded-tls	passed	00:53:43
upgrade-sharded	passed	00:20:33
upgrade-partial-backup	passed	00:16:13
users	passed	00:17:44
version-service	passed	00:25:08

Summary	Value
Tests Run	72/72
Job Duration	03:04:50
Total Test Time	21:27:18

commit: 8f87840
image: perconalab/percona-server-mongodb-operator:PR-2089-8f878403