Reclassify skip parameters from behavioral to functional

The 'skip' and 'skipMain' parameters directly affect build output:
- skip=true: No bytecode produced (compilation skipped entirely)
- skip=false: Bytecode produced normally

This is a functional difference (affects artifacts), not behavioral
(affects only how the build runs). Updated all three occurrences:
- compile goal: skip parameter
- compile goal: skipMain parameter
- testCompile goal: skip parameter

Addresses reviewer feedback on PR apache#389.

Author: cowwoc

src/main/resources/plugin-parameters/maven-compiler-plugin.xml

@@ -121,13 +121,13 @@
         </parameter>
         <parameter>
           <name>skip</name>
-          <type>behavioral</type>
-          <description>Skip compilation entirely</description>
+          <type>functional</type>
+          <description>Skip compilation entirely (no bytecode produced when true)</description>
         </parameter>
         <parameter>
           <name>skipMain</name>
-          <type>behavioral</type>
-          <description>Skip compiling main sources</description>
+          <type>functional</type>
+          <description>Skip compiling main sources (no main bytecode produced when true)</description>
         </parameter>
         <parameter>
           <name>failOnError</name>
@@ -273,8 +273,8 @@
         </parameter>
         <parameter>
           <name>skip</name>
-          <type>behavioral</type>
-          <description>Skip test compilation</description>
+          <type>functional</type>
+          <description>Skip test compilation (no test bytecode produced when true)</description>
         </parameter>
         <parameter>
           <name>failOnError</name>