fix: Clarify what has-root-span does (#1114)

Noted in pollinators:
https://honeycombpollinators.slack.com/archives/C01BZ3J7RMJ/p1714601065818559

---------

Co-authored-by: Kent Quirk <kentquirk@honeycomb.io>

rules_complete.yaml

@@ -108,6 +108,8 @@ Samplers:
                 - Name: drop incomplete traces from the buggy service
                   Drop: true
                   Conditions:
+                    # NOTE: has-root-span only checks if the trace has a root span.
+                    # It does NOT act as a shorthand for evaluating the root span; use the `root.` construct for that.
                     - Operator: has-root-span
                       Value: false
                     - Field: service.name

rules_conditions.md

@@ -201,6 +201,15 @@ For most cases, use `not-exists` in a rule with a scope of "span".
 WARNING: Rules can have `Scope: trace` or `Scope: span`; `not-exists` used with `Scope: trace` will be true if **any** single span in the entire trace matches the negative condition.
 This is almost never desired behavior.
 
+### has-root-span
+
+Tests if the trace as a whole has a root span.
+
+The `Value` parameter can either be `true` or `false`.
+
+NOTE: `has-root-span` does not check if a given span **is** a root span,
+it checks if the containing trace **has** a root span.
+
 ### `matches`
 
 Tests if the span value specified by the `Field` parameter matches the regular expression specified by the `Value` parameter.