GH-3108: Document that NamingConvention only processes dot-separated names

[won-seoop]

Summary

Closes #3108

The root confusion in this issue is that users expect NamingConvention.snakeCase to convert all separators (including hyphens) to underscores, but it only processes dot-separated names — which is Micrometer's canonical form.

The issue thread clarifies (from @shakuzen):

In general, we don't want to change things any more than we have to for a backend's requirements... NamingConvention.snakeCase is a general implementation not specific to any backend, and therefore it only handles converting from canonical form (dot case) to snake case.
PrometheusNamingConvention can be more specific... Prometheus doesn't allow many characters - so it is by chance that kebab case also gets converted, because - is not an allowed character.
The problem here is that NamingConvention.snakeCase should not be used with PrometheusMeterRegistry.

This PR adds a NOTE callout in the naming.adoc Tag Naming section that:

• Explains that generic naming conventions only translate dot separators, not hyphens or other chars
• Shows a concrete example (my-tag-name stays as my-tag-name, but my.tag.name → my_tag_name)
• Notes that each registry ships with a backend-specific naming convention, and replacing it (e.g., Prometheus' default) with NamingConvention.snakeCase produces unexpected results

Test plan

• ./gradlew antora generates the site and the Tag Naming section includes the new NOTE
• Example text renders correctly

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