[Docs] Add all chooseable cron config options to 'app.example.ini'

[a1012112796]

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

gitea/modules/setting/cron.go

Lines 7 to 11 in ba20dd7

	// GetCronSettings maps the cron subsection to the provided config
	func GetCronSettings(name string, config interface{}) (interface{}, error) {
	err := Cfg.Section("cron." + name).MapTo(config)
	return config, err
	}

[codecov-commenter]

Codecov Report

Merging #12931 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12931   +/-   ##
=======================================
  Coverage   42.81%   42.82%           
=======================================
  Files         661      661           
  Lines       72986    72986           
=======================================
+ Hits        31249    31253    +4     
  Misses      36663    36663           
+ Partials     5074     5070    -4     

Impacted Files	Coverage Δ
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
modules/log/event.go	57.54% <0.00%> (-1.89%)	⬇️
routers/repo/view.go	37.47% <0.00%> (-0.65%)	⬇️
services/pull/pull.go	40.78% <0.00%> (ø)
modules/git/repo.go	46.70% <0.00%> (+0.50%)	⬆️
services/pull/check.go	50.76% <0.00%> (+3.07%)	⬆️
modules/git/utils.go	77.04% <0.00%> (+3.27%)	⬆️
services/pull/patch.go	69.82% <0.00%> (+3.44%)	⬆️
modules/indexer/stats/db.go	60.86% <0.00%> (+8.69%)	⬆️

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[6543]

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