Enhance NGINX documentation to clarify that service specific NGINX settings should be configured explicitly and will not inherit from global one

Update: We decided not to set service specific nginx values based on global one, but make it explicit in the documentation that users have to separately set them.

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Original MR title: Set non-overriden service specific nginx values same as global nginx ones

Setting a value for nginx['foo'] in /etc/gitlab/gitlab.rb will not automatically make it reflect on service_nginx['foo']. IMO, that is a bug.

Consider the following, of how I think it should be, with pages as an example

default['gitlab']['nginx']['foo'] = 'DefaultRandomValue'

Then, node['gitlab']['pages-nginx']['foo'] should have values as the following table.

nginx['foo'] in gitlab.rb	pages_nginx['foo'] in gitlab.rb	node['gitlab']['nginx']['foo']	node['gitlab']['pages-nginx']['foo']
not set	not set	DefaultRandomValue	DefaultRandomValue
AnotherValue	not set	AnotherValue	AnotherValue
not set	DifferentValue	DefaultRandomValue	DifferentValue
AnotherValue	DifferentValue	AnotherValue	DifferentValue

Closes #3993 (closed)