[Doc] Add note that defining label for aggregation placeholder is required

[sjvrijn]

Documentation improvement suggestion based on my personal issues in finding this information, see #1404

[coveralls]

Pull Request Test Coverage Report for Build 3115

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 52.071%

---

Files with Coverage Reduction	New Missed Lines	%
lib/Munin/Master/UpdateWorker.pm	2	94.62%
<!--	Total:	2

Totals
Change from base Build 3096:	-0.1%
Covered Lines:	1081
Relevant Lines:	2076

---

💛 - Coveralls