Documentation: Update for Reservoir Sampling (#919)

* update option for reservoir sampling

* update for clarity rendering

docs/source/data_readers.rst

@@ -50,9 +50,11 @@ be specified using the options dict parameter.
 Possible `options`:
 
 * delimiter - Must be a string, for example `"delimiter": ","`
-* data_format - must be a string, possible choices: "dataframe", "records"
-* selected_columns - columns being selected from the entire dataset, must be a 
+* data_format - Must be a string, possible choices: "dataframe", "records"
+* selected_columns - Columns being selected from the entire dataset, must be a 
   list `["column 1", "ssn"]`
+* sample_nrows - Reservoir sampling to sample `"n"` rows out of a total of `"M"` rows.
+  Specified for how many rows to sample, default None.
 * header - Define the header, for example
 
   * `"header": 'auto'` for auto detection