chore: Improve Initcap test and docs (#2387)

Author: andygrove

docs/source/user-guide/latest/expressions.md

@@ -24,13 +24,13 @@ natively in Comet and provide the same results as Spark, or will fall back to Sp
 be compatible.
 
 All expressions are enabled by default, but can be disabled by setting
-`spark.comet.expression.EXPRNAME.enabled=false`, where `EXPRNAME` is the expression name as specified in 
+`spark.comet.expression.EXPRNAME.enabled=false`, where `EXPRNAME` is the expression name as specified in
 the following tables, such as `Length`, or `StartsWith`.
 
 Expressions that are not Spark-compatible will fall back to Spark by default and can be enabled by setting
 `spark.comet.expression.EXPRNAME.allowIncompatible=true`.
 
-It is also possible to specify `spark.comet.expression.allowIncompatible=true` to enable all 
+It is also possible to specify `spark.comet.expression.allowIncompatible=true` to enable all
 incompatible expressions.
 
 ## Conditional Expressions
@@ -68,7 +68,7 @@ incompatible expressions.
 | ConcatWs        | Yes               |                                                                                                            |
 | Contains        | Yes               |                                                                                                            |
 | EndsWith        | Yes               |                                                                                                            |
-| InitCap         | No                | Requires `spark.comet.exec.initCap.enabled=true`                                                           |
+| InitCap         | No                | Behavior is different in some cases, such as hyphenated names.                                             |
 | Length          | Yes               |                                                                                                            |
 | Like            | Yes               |                                                                                                            |
 | Lower           | No                | Results can vary depending on locale and character set. Requires `spark.comet.caseConversion.enabled=true` |

spark/src/test/scala/org/apache/comet/CometStringExpressionSuite.scala

@@ -95,14 +95,26 @@ class CometStringExpressionSuite extends CometTestBase {
     }
   }
 
-  test("InitCap") {
+  test("InitCap compatible cases") {
     val table = "names"
     withTable(table) {
       sql(s"create table $table(id int, name varchar(20)) using parquet")
-      sql(
-        s"insert into $table values(1, 'james smith'), (2, 'michael rose'), " +
-          "(3, 'robert williams'), (4, 'rames rose'), (5, 'james smith'), " +
-          "(6, 'robert rose-smith'), (7, 'james ähtäri')")
+      withSQLConf(CometConf.getExprAllowIncompatConfigKey("InitCap") -> "true") {
+        sql(
+          s"insert into $table values(1, 'james smith'), (2, 'michael rose'), " +
+            "(3, 'robert williams'), (4, 'rames rose'), (5, 'james smith'), " +
+            "(7, 'james ähtäri')")
+        checkSparkAnswerAndOperator(s"SELECT initcap(name) FROM $table")
+      }
+    }
+  }
+
+  test("InitCap incompatible cases") {
+    val table = "names"
+    withTable(table) {
+      sql(s"create table $table(id int, name varchar(20)) using parquet")
+      // Comet and Spark differ on hyphenated names
+      sql(s"insert into $table values(6, 'robert rose-smith')")
       checkSparkAnswer(s"SELECT initcap(name) FROM $table")
     }
   }