Support label with @Option

- Add `label` field into `@Option`. This sets option label if
  `label` field has any text.
- Fixes #732

spring-shell-docs/src/main/asciidoc/using-shell-options-label.adoc

@@ -7,15 +7,21 @@ what a default `help` command outputs. Within a command documentation
 a type of an option is documented but this is not always super useful. Thus
 you may want to give better descriptive word for an option.
 
-====
-[source, java, indent=0]
+NOTE: Label is not supported with `legacy annotation`.
+
+[source,java,indent=0,role="primary"]
+.Programmatic
 ----
-include::{snippets}/OptionSnippets.java[tag=option-registration-label]
+include::{snippets}/OptionSnippets.java[tag=option-label-programmatic]
+----
+
+[source,java,indent=0,role="secondary"]
+.Annotation
+----
+include::{snippets}/OptionSnippets.java[tag=option-label-annotation]
 ----
-====
 
 Defining label is then shown in `help`.
-
 ====
 [source, bash]
 ----

spring-shell-docs/src/test/java/org/springframework/shell/docs/OptionSnippets.java

@@ -372,6 +372,13 @@ public class OptionSnippets {
 		}
 		// end::option-default-annotation[]
 
+		// tag::option-label-annotation[]
+		void labelOption(
+			@Option(label = "MYLABEL") String arg
+		) {
+		}
+		// end::option-label-annotation[]
+
 	}
 
 	static class Registration {
@@ -436,5 +443,15 @@ public class OptionSnippets {
 		}
 		// end::option-default-programmatic[]
 
+		// tag::option-label-programmatic[]
+		CommandRegistration labelOption() {
+			return CommandRegistration.builder()
+				.withOption()
+					.longNames("arg")
+					.label("MYLABEL")
+					.and()
+				.build();
+		}
+		// end::option-label-programmatic[]
 	}
 }