Singular variant for plural string must contain @count

All plural APIs recommend write singular value with "1" value instead of "@count".

   * @param string $singular
   *   The string for the singular case. Make sure it is clear this is singular,
   *   to ease translation (e.g. use "1 new comment" instead of "1 new"). Do not
   *   use @count in the singular string.

Do not use @count in the singular string.

This line is misleading and can cause problems.

This recommendation can be found in core:

1. \Drupal\Core\StringTranslation\PluralTranslatableMarkup::__construct
2. \Drupal\Core\StringTranslation\TranslationInterface::formatPlural
3. Drupal.formatPlural (drupal.es6.js)

The problem is that not only "1" can have singular value in different languages.

E.g. in Russian "21", "31", "41" and so on is singular value: "51 товар" (51 products), but not "11".

This is handled by formula "Plural-Forms: nplurals=3; plural=((((n%10)==1)&&((n%100)!=11))?(0):(((((n%10)>=2)&&((n%10)<=4))&&(((n%100)<10)||((n%100)>=20)))?(1):2));\n".

The translation strings is:

msgid "1 product"
msgid_plural "@count products"
msgstr[0] "1 товар"
msgstr[1] "@count товара"
msgstr[2] "@count товаров"

So in that case new PluralTranslatableMarkup(21, '1 product', '@count products') will return "1 товар" (1 product).

The solution is easy — we need use "@count" for singular value as well. In that case problem is not exists. new PluralTranslatableMarkup(21, '@count product', '@count products') will return "21 товар" (21 products).

At least we need to change documentation and suggest to use "@count" for singular value. Also, it will require to change all plural strings.