fix(om2): clarify empty UNIT/HELP value wording (#2931)

Be more precise about where the empty value appears (before the newline)
and align with standard spec language.


Coded with Claude Sonnet 4.6.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

Author: krajorama

docs/specs/om/open_metrics_spec_2_0.md

@@ -727,7 +727,7 @@ It is also valid to have:
 
 If the unit is known it SHOULD be provided.
 
-The value of a UNIT or HELP line MAY be empty. This MUST be treated as if no metadata line for the MetricFamily existed.
+A UNIT or HELP metadata line MAY have an empty value string before the newline. This MUST be treated as if the metadata line were not present.
 
 Full example: