Precision about OptionParser 'separator' (#8274)

* Precision about OptionParser 'separator'

Clarified requirement for 'separator' option to require 'multiple' to be true.

* Fix grammar in option-parsers.md documentation

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Co-authored-by: Mark Scherer <dereuromark@users.noreply.github.com>

Author: Erwane

docs/en/console-commands/option-parsers.md

@@ -158,7 +158,7 @@ of the option:
 - `multiple` - The option can be provided multiple times. The parsed option
   will be an array of values when this option is enabled.
 - `separator` - A character sequence that the option value is split into an
-  array with.
+  array with. Requires `multiple` set to `true`.
 - `choices` - An array of valid choices for this option. If left empty all
   values are valid. An exception will be raised when parse() encounters an
   invalid value.