parser_multiline: correct the formatN parameter table

[Watson1978]

What

Fix the formatN parameter table on parser/multiline.md:

• default: `nil` → required parameter
• type: string → regexp

Why

The article states "It is a required parameter." right, yet the table listed nil as the default. A required parameter cannot have a default value, so the two descriptions contradicted each other.

formatN is also not declared as a config_param. In lib/fluent/plugin/parser_multiline.rb, only format_firstline and unmatched_lines are declared; format1 to format20 are read directly from the config element inside parse_formats. So the string / nil shown in the table had no basis in the config system.

formatN is validated as a regular expression by check_format_regexp.

      def check_format_regexp(format, key)
        if format[0] == '/' && format[-1] == '/'
          begin
            Regexp.new(format[1..-2], Regexp::MULTILINE)
          rescue => e
            raise Fluent::ConfigError, "Invalid regexp in #{key}: #{e}"
          end
        else
          raise Fluent::ConfigError, "format should be Regexp, need //, in #{key}: '#{format}'"
        end
      end

https://github.com/fluent/fluentd/blob/469d5c239113f36788046f6222d840aee8615047/lib/fluent/plugin/parser_multiline.rb#L139-L149

Note

This addresses the remaining point of #466. That issue raised two problems: the undocumented got incomplete line before first line warning (handled in #655) and this formatN table mismatch. #466 was closed after #655 was merged, so this PR references it for context only.

Refs #466