Replace zsh END_MARKER pseudo-completion with a space to ease migration.

Document why & how this pseudo-completion is used.

Signed-off-by: Ross Goldberg <[email protected]>

Author: rgoldberg

Sources/ArgumentParser/Completions/CompletionsGenerator.swift

@@ -102,7 +102,10 @@ public struct CompletionShell: RawRepresentable, Hashable, CaseIterable {
   func format(completions: [String]) -> String {
     var completions = completions
     if self == .zsh {
-      completions.append("END_MARKER")
+      // This pseudo-completion is removed by the zsh completion script.
+      // It allows trailing empty string completions to work in zsh.
+      // zsh completion scripts generated by older SAP versions ignore spaces.
+      completions.append(" ")
     }
     return completions.joined(separator: "\n")
   }