[php-langspec][Issue #232] - Describe list expression differently to remove some constraits

You can view the Issue on github: https://github.com/php/php-langspec/issues/232 Opened By: zhujinxuan Issue Description: Hi, I think it is perhaps better to document the list-expression in this way, which automatically ensures there is at least one valid element: ``` list-intrinsic: list ( list-expression-list ) list-expression-list: commas? list-element commas? commas? list-element , list-expression-list? commas: , , commas list-element expression => list-or-variable list-or-variable ``` Then we can remove the following constraints documentation: > At least one of the elements of the list-expression-list must be non-empty. I think this makes the document a bit neater and parser can write the list parser just by reading the expression.