Add docs for tablesample system_time()

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+
+
+
+ tsm_system_time
+
+ 
+  tsm_system_time
+ 
+
+ 
+  The tsm_system_time module provides the tablesample method
+  SYSTEM_TIME, which can be used inside the
+  TABLESAMPLE clause of a SELECT.
+ 
+
+ 
+  This tablesample method uses a linear probing algorithm to read sample
+  of a table and uses time in milliseconds as limit (unlike the
+  SYSTEM tablesample method which limits by percentage
+  of a table). This gives you some control over the length of execution
+  of your query.
+ 
+
+ 
+  Examples
+
+  
+   Here is an example of selecting sample of a table with
+   SYSTEM_TIME. First install the extension:
+  
+
+
+CREATE EXTENSION tsm_system_time;
+
+
+  
+   Then you can use it in a SELECT command the same way as
+   other tablesample methods:
+
+
+SELECT * FROM my_table TABLESAMPLE SYSTEM_TIME(1000);
+
+  
+
+  
+   The above command will return as large a sample of my_table as it can read in
+   1 second (or less if it reads whole table faster).
+  
+ 
+
+