Fix alias handling with @Command annotation

- Revisit how alias commands are added using
  @Command annotation when using if/or on class
  and/or method level.
- With this change alias handling is more logical
  and there's better tests and docs.
- Backport #945
- Fixes #973

spring-shell-docs/modules/ROOT/nav.adoc

@@ -13,6 +13,7 @@
 *** xref:commands/exceptionhandling/resolving.adoc[]
 *** xref:commands/exceptionhandling/mappings.adoc[]
 *** xref:commands/exceptionhandling/annotation.adoc[]
+** xref:commands/alias.adoc[]
 ** xref:commands/hidden.adoc[]
 ** xref:commands/helpoptions.adoc[]
 ** xref:commands/interactionmode.adoc[]