Add db/docs descriptions for sharding team

Updates the db/docs YML files with descriptions for each table.

db/docs/ci_namespace_mirrors.yml

@@ -4,6 +4,6 @@ classes:
 - Ci::NamespaceMirror
 feature_categories:
 - sharding
-description: TODO
+description: Mirrors some data from the `main` database into the `ci` database so that we can join directly in a single query
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75621
 milestone: '14.6'

db/docs/ci_project_mirrors.yml

@@ -4,6 +4,6 @@ classes:
 - Ci::ProjectMirror
 feature_categories:
 - sharding
-description: TODO
+description: Mirrors some data from the `main` database into the `ci` database so that we can join directly in a single query
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75621
 milestone: '14.6'

db/docs/loose_foreign_keys_deleted_records.yml

@@ -4,6 +4,6 @@ classes:
 - LooseForeignKeys::DeletedRecord
 feature_categories:
 - sharding
-description: TODO
+description: Used by the loose foreign keys feature as a queue of parent records whose child records (via foreign keys) need to be deleted/nullified
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70152
 milestone: '14.3'

db/docs/namespaces_sync_events.yml

@@ -4,6 +4,6 @@ classes:
 - Namespaces::SyncEvent
 feature_categories:
 - sharding
-description: TODO
+description: Used as a queue of data that needs to be synchronized between the `ci` and `main` database
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75517
 milestone: '14.6'

db/docs/projects_sync_events.yml

@@ -4,6 +4,6 @@ classes:
 - Projects::SyncEvent
 feature_categories:
 - sharding
-description: TODO
+description: Used as a queue of data that needs to be synchronized between the `ci` and `main` database
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75517
 milestone: '14.6'