[Docs] Add all chooseable cron config options to 'app.example.ini' (#12931)

As title.

ref:
https://github.com/go-gitea/gitea/blob/master/modules/cron/tasks_basic.go
https://github.com/go-gitea/gitea/blob/master/modules/cron/tasks_extended.go
ba20dd7a7b/modules/setting/cron.go (L7-L11)

Signed-off-by: a1012112796 <1012112796@qq.com>

custom/conf/app.example.ini

@@ -889,13 +889,27 @@ ENABLED = true
 ; Run cron tasks when Gitea starts.
 RUN_AT_START = false
 
+; Basic cron tasks
+
 ; Update mirrors
 [cron.update_mirrors]
 SCHEDULE = @every 10m
+; Enable running Update mirrors task periodically.
+ENABLED = true
+; Run Update mirrors task when Gitea starts.
+RUN_AT_START = false
+; Notice if not success
+NO_SUCCESS_NOTICE = true
 
 ; Repository health check
 [cron.repo_health_check]
 SCHEDULE = @every 24h
+; Enable running Repository health check task periodically.
+ENABLED = true
+; Run Repository health check task when Gitea starts.
+RUN_AT_START = false
+; Notice if not success
+NO_SUCCESS_NOTICE = true
 TIMEOUT = 60s
 ; Arguments for command 'git fsck', e.g. "--unreachable --tags"
 ; see more on http://git-scm.com/docs/git-fsck
@@ -903,7 +917,12 @@ ARGS =
 
 ; Check repository statistics
 [cron.check_repo_stats]
+; Enable running check repository statistics task periodically.
+ENABLED = true
+; Run check repository statistics task when Gitea starts.
 RUN_AT_START = true
+; Notice if not success
+NO_SUCCESS_NOTICE = true
 SCHEDULE = @every 24h
 
 ; Clean up old repository archives
@@ -912,6 +931,8 @@ SCHEDULE = @every 24h
 ENABLED = true
 ; Whether to always run at least once at start up time (if ENABLED)
 RUN_AT_START = true
+; Notice if not success
+NO_SUCCESS_NOTICE = true
 ; Time interval for job to run
 SCHEDULE = @every 24h
 ; Archives created more than OLDER_THAN ago are subject to deletion
@@ -919,19 +940,103 @@ OLDER_THAN = 24h
 
 ; Synchronize external user data (only LDAP user synchronization is supported)
 [cron.sync_external_users]
+ENABLED = true
 ; Synchronize external user data when starting server (default false)
 RUN_AT_START = false
+; Notice if not success
+NO_SUCCESS_NOTICE = true
 ; Interval as a duration between each synchronization (default every 24h)
 SCHEDULE = @every 24h
 ; Create new users, update existing user data and disable users that are not in external source anymore (default)
 ;   or only create new users if UPDATE_EXISTING is set to false
 UPDATE_EXISTING = true
 
-; Update migrated repositories' issues and comments' posterid, it will always attempt synchronization when the instance starts.
+; Clean-up deleted branches
+[cron.deleted_branches_cleanup]
+ENABLED = true
+; Clean-up deleted branches when starting server (default true)
+RUN_AT_START = true
+; Notice if not success
+NO_SUCCESS_NOTICE = true
+; Interval as a duration between each synchronization (default every 24h)
+SCHEDULE = @every 24h
+; deleted branches than OLDER_THAN ago are subject to deletion
+OLDER_THAN = 24h
+
 [cron.update_migration_poster_id]
+; Update migrated repositories' issues and comments' posterid, it will always attempt synchronization when the instance starts.
+ENABLED = true
+; Update migrated repositories' issues and comments' posterid when starting server (default true)
+RUN_AT_START = true
+; Notice if not success
+NO_SUCCESS_NOTICE = true
 ; Interval as a duration between each synchronization. (default every 24h)
 SCHEDULE = @every 24h
 
+; Extened cron task
+; they was not enabled as default
+
+; Delete all unactivated accounts
+[cron.delete_inactive_accounts]
+ENABLED = false
+RUN_AT_START = false
+NO_SUCCESS_NOTICE = true
+SCHEDULE = @annually
+OLDER_THAN = 168h
+
+; Delete all repository archives
+[cron.delete_repo_archives]
+ENABLED = false
+RUN_AT_START = false
+NO_SUCCESS_NOTICE = true
+SCHEDULE = @annually
+
+; Garbage collect all repositories
+[cron.git_gc_repos]
+ENABLED = false
+RUN_AT_START = false
+NO_SUCCESS_NOTICE = true
+SCHEDULE = @every 72h
+TIMEOUT = 60s
+; Arguments for command 'git gc'
+; The default value is same with [git] -> GC_ARGS
+ARGS =
+
+; Update the '.ssh/authorized_keys' file with Gitea SSH keys
+[cron.resync_all_sshkeys]
+ENABLED = false
+RUN_AT_START = false
+NO_SUCCESS_NOTICE = true
+SCHEDULE = @every 72h
+
+; Resynchronize pre-receive, update and post-receive hooks of all repositories.
+[cron.resync_all_hooks]
+ENABLED = false
+RUN_AT_START = false
+NO_SUCCESS_NOTICE = true
+SCHEDULE = @every 72h
+
+; Reinitialize all missing Git repositories for which records exist
+[cron.reinit_missing_repos]
+ENABLED = false
+RUN_AT_START = false
+NO_SUCCESS_NOTICE = true
+SCHEDULE = @every 72h
+
+; Delete all repositories missing their Git files
+[cron.delete_missing_repos]
+ENABLED = false
+RUN_AT_START = false
+NO_SUCCESS_NOTICE = true
+SCHEDULE = @every 72h
+
+; Delete generated repository avatars
+[cron.delete_generated_repository_avatars]
+ENABLED = false
+RUN_AT_START = false
+NO_SUCCESS_NOTICE = true
+SCHEDULE = @every 72h
+
 [git]
 ; The path of git executable. If empty, Gitea searches through the PATH environment.
 PATH =