SPARK-37981 Add note for deleting Null and NaN

### What changes were proposed in this pull request?

This is for SPARK-37981Deletes columns with all Null as default.
Do also see #26098
User HyukjinKwon did a reviewed on 21 Oct 2019 
"Hey, you should document this in DataFrameWrtier, DataStreamWrtier, readwriter.py"

### Why are the changes needed?
Users need to know why there column(s) with all NaN or Null are gone. 

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
No.

python/pyspark/pandas/generic.py

@@ -904,6 +904,8 @@ def to_json(
 
         .. note:: output JSON format is different from pandas'. It always use `orient='records'`
             for its output. This behaviour might have to change in the near future.
+
+        .. note:: If column have only NaN or Null values. The column well be deleted.
 
         Note NaN's and None will be converted to null and datetime objects
         will be converted to UNIX timestamps.