Reword uses of "iff" in completions code.

Redid a comment as a DocC.

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

Author: rgoldberg

Sources/ArgumentParser/Completions/CompletionsGenerator.swift

@@ -173,8 +173,9 @@ extension ParsableCommand {
 }
 
 extension [ParsableCommand.Type] {
-  // Include default 'help' subcommand in nonempty subcommand list iff no help subcommand already exists.
-  mutating func addHelpSubcommandIffMissing() {
+  /// Include default 'help' subcommand in nonempty subcommand list if & only if
+  /// no help subcommand already exists.
+  mutating func addHelpSubcommandIfMissing() {
     if !isEmpty && allSatisfy({ $0._commandName != "help" }) {
       append(HelpCommand.self)
     }

Sources/ArgumentParser/Completions/ZshCompletionsGenerator.swift

@@ -55,7 +55,7 @@ extension [ParsableCommand.Type] {
       argumentSpecs.append("'(-)*:: :->arg'")
 
       if isRootCommand {
-        subcommands.addHelpSubcommandIffMissing()
+        subcommands.addHelpSubcommandIfMissing()
       }
 
       subcommandHandler = """