Constructs new doc comment by appending
custom doc tags (@block, @elem, @mod) to
to source doc comment.

@param {Object} node AST node to document
@param {Object} entity BEM entity, declared by node
@returns {String} doc comment for a bem entity
function getEntityDocComment(node, entity) {
    var comment = ['/**'];
    if (node.leadingComments && node.leadingComments[0]) {
        comment.push(node.leadingComments[0].value);
    }

    if (entity.block) {
        comment.push('@block');
    }

    if (entity.mod) {
        comment.push('@mod');
    }

    if (entity.val) {
        comment.push('@val');
    }

    comment.push('*/');
    return comment.join('\n');
}