Improve documentation of resource_attributes_as_tags

[mackjmr]

Description:
This PR improves documentation of resource_attributes_as_tags.

Link to tracking Issue:
Closes #29702

Testing:

Documentation:

[ringerc]

Thanks. The docs link should be significantly helpful.

It wouldn't hurt to have an example too, say

For example, host.name will always be mapped to datadog tag "host" by the exporter and service.name will be mapped to datadog tag "service". If this setting is "true", the original "host.name" and "service.name" tags will ALSO be retained and sent verbatim to datadog.

Consider warning the customer about the impact of this on DD billing by linking to the custom metrics billing docs page. Enabling this setting without careful filtering of resource attributes could result in bill shock.