[FAB-4931] Update doc for core.yaml logging section
[FAB-4931] Update doc for core.yaml logging section

This CR updates the comments for the logging section of core.yaml
to accurately document the usage of the available environment variables.

Change-Id: Ib042b9af1deb925d8fa30b870b7c774906909564
Signed-off-by: Will Lahti <f19a2779710675aad3df5c81e4ecfde11707e3aa@us.ibm.com>

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