Update DRUG_EXPOSURE.md

added visit_detail_id
https://github.com/OHDSI/CommonDataModel/issues/70

Documentation/CommonDataModel_Wiki_Files/StandardizedClinicalDataTables/DRUG_EXPOSURE.md

@@ -31,6 +31,7 @@ Field|Required|Type|Description
 |drug_source_concept_id|No|integer|A foreign key to a Drug Concept that refers to the code used in the source.|
 |route_source_value|No|varchar(50)|The information about the route of administration as detailed in the source.|
 |dose_unit_source_value|No|varchar(50)|The information about the dose unit as detailed in the source.|
+|visit_detail_id|No|integer|A foreign key to the visit in the VISIT_DETAIL table during which the Drug Exposure was initiated.|
 
 ### Conventions 
 
@@ -43,4 +44,4 @@ Field|Required|Type|Description
   * The lot_number field contains an identifier assigned from the manufacturer of the Drug product. 
   * If possible, the visit in which the drug was prescribed or delivered is recorded in the visit_occurrence_id field through a reference to the visit table.
   * If possible, the prescribing or administering provider (physician or nurse) is recorded in the provider_id field through a reference to the provider table.
-  * The drug_exposure_end_date denotes the day the drug exposure ended for the patient. This could be that the duration of drug_supply was reached (in which case drug_exposure_end_date = drug_exposure_start_date + days_supply -1), or because the exposure was stopped (medication changed, medication discontinued, etc.)
+  * The drug_exposure_end_date denotes the day the drug exposure ended for the patient. This could be that the duration of drug_supply was reached (in which case drug_exposure_end_date = drug_exposure_start_date + days_supply -1), or because the exposure was stopped (medication changed, medication discontinued, etc.)