Update udf-compiler descriptions in related docs

Signed-off-by: Allen Xu <allxu@nvidia.com>

docs/compatibility.md

@@ -290,10 +290,10 @@ Casting from string to timestamp currently has the following limitations.
  Only timezone 'Z' (UTC) is supported. Casting unsupported formats will result in null values. 
 
 ## UDF to Catalyst Expressions
-To speedup the process of UDF, spark-rapids introduces a udf-compiler extension to translate UDFs to Catalyst expressions.
+To speedup the process of UDF, spark-rapids introduces a udf-compiler extension to translate UDFs to Catalyst expressions. This compiler will be injected automatically to spark extensions by setting `spark.plugins=com.nvidia.spark.SQLPlugin` and is disabled by default.
 
 To enable this operation on the GPU, set
-[`spark.rapids.sql.udfCompiler.enabled`](configs.md#sql.udfCompiler.enabled) to `true`, and `spark.sql.extensions=com.nvidia.spark.udf.Plugin`.
+[`spark.rapids.sql.udfCompiler.enabled`](configs.md#sql.udfCompiler.enabled) to `true`.
 
 However, Spark may produce different results for a compiled udf and the non-compiled. For example: a udf of `x/y` where `y` happens to be `0`, the compiled catalyst expressions will return `NULL` while the original udf would fail  the entire job with a `java.lang.ArithmeticException: / by zero`
 

udf-compiler/README.md

@@ -14,6 +14,6 @@ How to run
 ----------
 
 The UDF compiler is included in the rapids-4-spark jar that is produced by the `dist` maven project.  Set up your cluster to run the RAPIDS Accelerator for Apache Spark
-and set the spark config `spark.sql.extensions` to include `com.nvidia.spark.udf.Plugin`.
+and this udf plugin will be automatically injected to spark extensions when `com.nvidia.spark.SQLPlugin` is set.
 
 The plugin is still disabled by default and you will need to set `spark.rapids.sql.udfCompiler.enabled` to `true` to enable it.