AutoComplete.GenerateCompletion generated man page summary improvement

[remkop]

The generated man page for generate-completion (AutoComplete.GenerateCompletion) in the picocli documentation (build\picocli-generated-docs\generate-completion.adoc) has the following summary:

generate-completion - Generate bash/zsh completion script for generate-completion.

During the picocli 4.4 release, I manually changed this to the following:

generate-completion - Generate bash/zsh completion script for the root command of this command.

See if we can automate this.

---

One idea is to create a command in picocli-codegen that has @Command(name = "the root command of this command"), and change picocli-codegen/build.gradle task generateManpageAsciiDoc to point to this command instead of to picocli.AutoComplete$GenerateCompletion.