Fix Scaladoc warnings in GpuScalaUDF and BufferSendState

[jlowe]

Fixes the following Scaladoc warnings that appear during the build:

[INFO] --- scala-maven-plugin:4.3.0:doc-jar (attach-scaladocs) @ rapids-4-spark-sql_2.12 ---
model contains 913 documentable templates
/spark-rapids/sql-plugin/src/main/scala/org/apache/spark/sql/rapids/GpuScalaUDF.scala:70: warning: Could not find any member to link for "RapidsUDF".
  /**
  ^
/spark-rapids/sql-plugin/src/main/scala/com/nvidia/spark/rapids/shuffle/BufferSendState.scala:154: warning: Could not find any member to link for "`RapidsShuffleSendPrepareException`".
  /**
  ^
two warnings found

[jlowe]

build