[DOCS][SQL] Add a Note on jsonFile having separate JSON objects per line

* This commit hopes to avoid the confusion I faced when trying
  to submit a regular, valid multi-line JSON file, also see

  http://apache-spark-user-list.1001560.n3.nabble.com/Loading-JSON-Dataset-fails-with-com-fasterxml-jackson-databind-JsonMappingException-td20041.html

Author: Peter Vandenabeele <peter@vandenabeele.com>

Closes #3517 from petervandenabeele/pv-docs-note-on-jsonFile-format/01 and squashes the following commits:

1f98e52 [Peter Vandenabeele] Revert to people.json and simple Note text
6b6e062 [Peter Vandenabeele] Change the "JSON" connotation to "txt"
fca7dfb [Peter Vandenabeele] Add a Note on jsonFile having separate JSON objects per line

docs/sql-programming-guide.md

@@ -625,6 +625,10 @@ This conversion can be done using one of two methods in a SQLContext:
 * `jsonFile` - loads data from a directory of JSON files where each line of the files is a JSON object.
 * `jsonRDD` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
 
+Note that the file that is offered as _jsonFile_ is not a typical JSON file. Each
+line must contain a separate, self-contained valid JSON object. As a consequence,
+a regular multi-line JSON file will most often fail.
+
 {% highlight scala %}
 // sc is an existing SparkContext.
 val sqlContext = new org.apache.spark.sql.SQLContext(sc)
@@ -663,6 +667,10 @@ This conversion can be done using one of two methods in a JavaSQLContext :
 * `jsonFile` - loads data from a directory of JSON files where each line of the files is a JSON object.
 * `jsonRDD` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
 
+Note that the file that is offered as _jsonFile_ is not a typical JSON file. Each
+line must contain a separate, self-contained valid JSON object. As a consequence,
+a regular multi-line JSON file will most often fail.
+
 {% highlight java %}
 // sc is an existing JavaSparkContext.
 JavaSQLContext sqlContext = new org.apache.spark.sql.api.java.JavaSQLContext(sc);
@@ -701,6 +709,10 @@ This conversion can be done using one of two methods in a SQLContext:
 * `jsonFile` - loads data from a directory of JSON files where each line of the files is a JSON object.
 * `jsonRDD` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
 
+Note that the file that is offered as _jsonFile_ is not a typical JSON file. Each
+line must contain a separate, self-contained valid JSON object. As a consequence,
+a regular multi-line JSON file will most often fail.
+
 {% highlight python %}
 # sc is an existing SparkContext.
 from pyspark.sql import SQLContext