[#280] fallbackValue: move content in user manual

docs/index.adoc

@@ -795,12 +795,48 @@ public interface IDefaultValueProvider {
 [#fallbackValue-annotation]
 === `fallbackValue` Annotation
 
-For options with an optional parameter, the fallback value (introduced in picocli 4.0) is assigned
-if the option is specified on the command line without an option parameter.
+If an option is defined with `arity = "0..1"`, it may or not have a parameter value.
+If such an option is specified without a value on the command line, it is assigned the fallback value.
+
+The `fallbackValue` annotation was introduced in picocli 4.0; prior to this, (from picocli 2.3) an empty String was assigned.
 
 This is different from the `defaultValue`, which is assigned if the option is not specified at all on the command line.
 
-See <<Optional Values>> for details.
+For example:
+
+[source, java]
+----
+class FallbackValueDemo implements Runnable {
+    @Option(names = "-x", arity = "0..1",
+            defaultValue = "-1", fallbackValue = "-2",
+            description = "Optional parameter. Default: ${DEFAULT-VALUE}, " +
+                          "if specified without parameter: ${FALLBACK-VALUE}")
+    int x;
+
+    public void run() { System.out.printf("x = %s%n", x); }
+
+    public static void main(String... args) {
+       CommandLine.run(new FallbackValueDemo(), args);
+    }
+}
+----
+Gives the following results:
+[source, bash]
+----
+java FallbackValueDemo -x 100
+x = 100
+
+java FallbackValueDemo -x
+x = -2
+
+java FallbackValueDemo
+x = -1
+----
+
+Any String value is converted to the type of the option before it is assigned to the option. Options and positional parameters may define a <<Custom Type Converters,custom type converter>> if necessary.
+
+Note that the option description may contain the `${FALLBACK-VALUE}` <<Predefined Variables,variable>> which will be replaced with the actual fallback value when the usage help is shown.
+
 
 == Multiple Values
 Multi-valued options and positional parameters are annotated fields that can capture multiple values from the command line.
@@ -944,42 +980,6 @@ When a `@Parameters` field is applied (because its index matches the index of th
 === Optional Values
 If an option is defined with `arity = "0..1"`, it may or not have a parameter value.
 If such an option is specified without a value on the command line, it is assigned the <<fallbackValue-annotation,fallback value>>.
-The `fallbackValue` annotation was introduced in picocli 4.0; prior to this, (from picocli 2.3) an empty String was assigned.
-
-If the option is not specified on the command line at all, it keeps its default value. For example:
-
-[source, java]
-----
-class OptionalValueDemo implements Runnable {
-    @Option(names = "-x", arity = "0..1",
-            defaultValue = "-1", fallbackValue = "-2",
-            description = "Optional parameter. Default: ${DEFAULT-VALUE}, " +
-                          "if specified without parameter: ${FALLBACK-VALUE}")
-    int x;
-
-    public void run() { System.out.printf("x = %s%n", x); }
-
-    public static void main(String... args) {
-       CommandLine.run(new OptionalValueDemo(), args);
-    }
-}
-----
-Gives the following results:
-[source, bash]
-----
-java OptionalValueDemo -x 100
-x = 100
-
-java OptionalValueDemo -x
-x = -2
-
-java OptionalValueDemo
-x = -1
-----
-
-Any String value is converted to the type of the option before it is assigned to the option. Options and positional parameters may define a <<Custom Type Converters,custom type converter>> if necessary.
-
-Note that the option description may contain the `${FALLBACK-VALUE}` <<Predefined Variables,variable>> which will be replaced with the actual fallback value when the usage help is shown.
 
 == Required Arguments
 === Required Options