config-labels: Add clarification about label inheritence
config-labels: Add clarification about label inheritence

Clarify that when modifying an inherited label, all its properties
must be redefined.

See [1] for an example of problems occurring due to this limitation
not being well documented.

[1] https://bugs.eclipse.org/bugs/show_bug.cgi?id=512468

Change-Id: I0f809e197b908af73cfd42d7d8e64e62dcb3167e

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