v2 feature: Use app.Description in templates

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Checklist

• Are you running the latest v2 release? The list of releases is here.
• Did you check the manual for your release? The v2 manual is here
• Did you perform a search about this feature? Here's the Github guide about searching.

What problem does this solve?

I was adding a .Description value to my main application, and I was surprised to find the text is not included in the output during man-page and markdown generation. Instead, the templates put App.UsageText in a section titled DESCRIPTION. Was this deliberate? If so, what is the difference between App.UsageText and App.Description? I assumed App.Description should be printed in the DESCRIPTION section, while the App.UsageText value is printed in place of the generated Usage Text:

{{ .App.Name }} [GLOBAL OPTIONS] command [COMMAND OPTIONS] [ARGUMENTS...]

Solution description

I believe a diff similar to the following would enable both UsageText and Description output:

diff --git a/template.go b/template.go
index 69e040e..e0b8a6b 100644
--- a/template.go
+++ b/template.go
@@ -86,15 +86,19 @@ var MarkdownDocTemplate = `% {{ .App.Name }} {{ .SectionNum }}
 {{ if .SynopsisArgs }}
 ` + "```" + `
 {{ range $v := .SynopsisArgs }}{{ $v }}{{ end }}` + "```" + `
-{{ end }}{{ if .App.UsageText }}
+{{ end }}{{ if .App.Description }}
 # DESCRIPTION
 
-{{ .App.UsageText }}
+{{ .App.Description }}
 {{ end }}
 **Usage**:
 
 ` + "```" + `
+{{ if .App.UsageText }}
+{{ .App.UsageText }}
+{{ else }}
 {{ .App.Name }} [GLOBAL OPTIONS] command [COMMAND OPTIONS] [ARGUMENTS...]
+{{ end }}
 ` + "```" + `
 {{ if .GlobalArgs }}
 # GLOBAL OPTIONS

Describe alternatives you've considered

I don't like putting my long-form description text in the UsageText field. Description has some nice formatting during console printing, and UsageText does not.