Github user lins05 commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15487#discussion_r83551685
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Last.scala
 ---
    @@ -29,15 +29,18 @@ import org.apache.spark.sql.types._
      * a single partition, and we use a single reducer to do the aggregation.).
      */
     @ExpressionDescription(
    -  usage = "_FUNC_(expr,isIgnoreNull) - Returns the last value of `child` 
for a group of rows.")
    +  usage = """_FUNC_(expr,isIgnoreNull) - Returns the last value of `child` 
for a group of rows.
    +    _FUNC_(expr,isIgnoreNull=false) - Returns the last value of `child` 
for a group of rows.
    --- End diff --
    
    @hvanhovell @HyukjinKwon Thanks for the review. I'm simply following the 
usage string of other functions, e.g:
    
    ```
    spark-sql> describe function first;
    Function: first
    Class: org.apache.spark.sql.catalyst.expressions.aggregate.First
    Usage: first(expr) - Returns the first value of `child` for a group of rows.
        first(expr,isIgnoreNull=false) - Returns the first value of `child` for 
a group of rows.
          If isIgnoreNull is true, returns only non-null values.
       
    spark-sql> describe function approx_count_distinct;
    Function: approx_count_distinct
    Class: 
org.apache.spark.sql.catalyst.expressions.aggregate.HyperLogLogPlusPlus
    Usage: approx_count_distinct(expr) - Returns the estimated cardinality by 
HyperLogLog++.
        approx_count_distinct(expr, relativeSD=0.05) - Returns the estimated 
cardinality by HyperLogLog++
          with relativeSD, the maximum estimation error allowed.
    ```
    
    So it seems the current convention is that: the first line is a short 
one-line description, followed by a detail description. Do we have any explicit 
"usage string style" to follow?
    
    @hvanhovell I'll add the note about nondeterministic.


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