[SPARK-25036][SQL] Avoid discarding unmoored doc comment in Scala-2.12.

[kiszk]

What changes were proposed in this pull request?

This PR avoid the following compilation error using sbt in Scala-2.12.

[error] [warn] /home/ishizaki/Spark/PR/scala212/spark/mllib/src/main/scala/org/apache/spark/ml/tree/impl/RandomForest.scala:410: discarding unmoored doc comment
[error] [warn]     /**
[error] [warn] 
[error] [warn] /home/ishizaki/Spark/PR/scala212/spark/mllib/src/main/scala/org/apache/spark/ml/tree/impl/RandomForest.scala:441: discarding unmoored doc comment
[error] [warn]     /**
[error] [warn] 
...
[error] [warn] /home/ishizaki/Spark/PR/scala212/spark/resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala:440: discarding unmoored doc comment
[error] [warn]     /**
[error] [warn] 

How was this patch tested?

Existing UTs

[SparkQA]

Test build #94511 has finished for PR 22059 at commit 70ec11e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[kiszk]

retest this please

[SparkQA]

Test build #94520 has finished for PR 22059 at commit 70ec11e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[kiszk]

cc @srowen @ueshin @HyukjinKwon

[srowen]

BTW I was working on compile errors too -- see srowen@5ea659f . You might see about picking up those into one PR rather than many. In particular I am interested to fix the ExprValue compile warning.

[HyukjinKwon]

Yea, It would be better to put those error into fewer PRs if possible.

[srowen]

Merged to master