</replaceable> in parentesis is not usual on DOCs

This page has 3 items that are between parentheses, there is an explanation
why they are used this way ?

https://www.postgresql.org/docs/devel/functions-json.html#FUNCTIONS-SQLJSON-TABLE

Each syntax element is described below in more detail.
*context_item*, *path_expression* [ AS *json_path_name* ] [ PASSING {
*value* AS *varname* } [, ...]]

The input data to query (*context_item*), the JSON path expression defining
the query (*path_expression*) with an optional name (*json_path_name*), and
an optional PASSING clause, which can provide data

Why (*context_item*), (*path_expression*) and (*json_path_name*) are inside
a parentheses ? This is not usual when explaining any other feature.

regards
Marcos