Add docs about validation of ILM policy

[jsoriano]

Add troubleshooting docs for validation added in elastic/package-spec#413.

[elasticmachine]

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• Start Time: 2022-09-13T15:15:19.124+0000

• Duration: 27 min 26 sec

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Test	Results
Failed	0
Passed	829
Skipped	0
Total	829

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (34/34)	💚
Files	66.929% (85/127)	👍
Classes	61.667% (111/180)	👍
Methods	48.834% (356/729)	👍
Lines	32.231% (3225/10006)	👍
Conditionals	100.0% (0/0)	💚