Updated descriptions and conventions

Updated based on OHDSI/WebAPI#234 (comment)

Author: gowthamrao

Documentation/CommonDataModel_Wiki_Files/StandardizedDerivedElements/COHORT.md

@@ -1,4 +1,4 @@
-The COHORT table contains records of subjects that satisfy a given set of criteria for a duration of time. The definition of the cohort is contained within the COHORT_DEFINITION table. Cohorts can be constructed of patients (Persons), Providers or Visits.
+The COHORT table contains records of subjects that satisfy a given set of criteria for a duration of time. The definition of the cohort is contained within the COHORT_DEFINITION table. Cohorts can be constructed of patients (Persons), Providers or Visits.  
 
 Field|Required|Type|Description
 :--------------------|:--------|:------------|:----------------------------
@@ -11,4 +11,6 @@ Field|Required|Type|Description
   * The core of a Cohort is the unifying definition or feature of the Cohort. This is captured in the cohort_definition_id. For example, Cohorts can include patients diagnosed with a specific condition, patients exposed to a particular drug, or Providers who have performed a specific Procedure.
   * Cohort records must have a Start Date
   * Cohort records must have an End Date, but may be set to Start Date or could have applied a censored date using the Observation Period Start Date.
-  * Cohort records must contain a Subject Id, which can refer to the Person, Provider, or Visit record. The Cohort Definition will define the type of subject through the subject concept id.
+  * Cohort records must contain a Subject Id, which can refer to the Person, Provider, Visit record or Care Site. The Cohort Definition will define the type of subject through the subject concept id.
+  * A subject can belong (or not belong) to a cohort at any moment in time
+  * A subject can only have one record in the cohort table for any moment of time, i.e. it is not possible for a person to contain multiple records indicating cohort membership that are overlapping in time