Update Periodicity.md

Author: usa-m

docs/usage/Periodicity.md

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+## Periodicity
+
+DDS.Net.Connector.Types.Enumerations.**Periodicity** specifies the periodicity for data updates. Its enumeration includes following types:
+
+  * *OnChange* - only when data value is changed
+  * *Highest* - update is done every *BASE_TIME_SLOT_MS* x 1 millisecond
+  * *High* - update is done every *BASE_TIME_SLOT_MS* x 2 millisecond
+  * *Normal* - update is done every *BASE_TIME_SLOT_MS* x 4 millisecond
+  * *Low* - update is done every *BASE_TIME_SLOT_MS* x 8 millisecond
+  * *Lowest* - update is done every *BASE_TIME_SLOT_MS* x 16 millisecond
+
+> *BASE_TIME_SLOT_MS* by default is set to *50*.
+