Document custom error payload fields

Related dev. issue(s): https://github.com/tarantool/tarantool/issues/9104

Product: Tarantool
Since: 3.1
Root document:

• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_error/new/
• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_error/error/

SME: @ Gumix

Details

Since Tarantool 3.1 it is possible to add a custom payload to the error on
error construction. The payload is passed as key-value pairs, where key
is a string, and value is any Lua object. The key name can be arbitrary
except it should not be the same as any of the built-in error field name:
reason, code, type, base_type, custom_type, errno, message, prev, trace.

Replace:

box.error.new({ reason = string[, code = number, type = string] }])

with:

box.error.new({ reason = string[, code = number, type = string, key1 = value1, ...] }])

Replace:

box.error({ reason = string[, code = number, type = string] }])

with:

box.error({ reason = string[, code = number, type = string, key1 = value1, ...] }])

Requested by @ Gumix in https://github.com/tarantool/tarantool/commit/eb2c6a4a4de8a958f07e5d4efc4453ded1c3026d.

Start with the two referenced box_error.new and box_error documentation pages. Update the displayed argument forms to include custom key-value payload fields and document the reserved built-in field names listed in the issue. Done means both root documents describe the Tarantool 3.1 behavior consistently.