update kube_node_status_{capacity/allocatable} doc to clarify difference

[exitflynn]

What this PR does / why we need it: The current description for kube_node_status_capacity and kube_node_status_allocatable uses the terms capacity and allocatable. What differentiates them is not clear from the description and is a cause of confusion to those who haven't encountered these terms before and are unaware of their definition in this context.

How does this change affect the cardinality of KSM: does not change cardinality

Which issue(s) this PR fixes :
Fixes #1859

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• ✅ login: exitflynn / name: Akshit Tyagi (837f747)

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This pull-request has been approved by: dgrisonnet, exitflynn

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