Add mention of exclusivity to end data API parameter

metobsapi/templates/data_index.html

@@ -184,7 +184,8 @@
                                 End of the query interval in UTC as YYYY-MM-DDTHH:MM:SS or -HH:MM:SS.
                                 If the format -HH:MM:SS is used, end is interpreted as a relative time
                                 starting from the current time. If end is not specified, it defaults to
-                                the current time.
+                                the current time. This time is exclusive meaning records exactly at this
+                                time will not be included in the results.
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