Add IsError worked example to Parameters docs (ticket 019)

Shows how to declare a boolean IsError param (regex (?i)error, kind
boolean, no capture group) that's true/false on every log line, using
the boolean param kind added in ticket 018.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/Docs.tsx

@@ -178,6 +178,14 @@ const SECTIONS: DocSection[] = [
           a metric (see <em>Filtering</em> above — bare params aren't queryable by name on their own, only metric
           labels/values are).
         </p>
+        <p>
+          Given the same kind of lines, declaring a param named <Code>IsError</Code> with regex{' '}
+          <Code>{String.raw`(?i)error`}</Code> and type <Code>boolean</Code> — no capture group needed — shows{' '}
+          <Code>IsError</Code> as <Code>true</Code> or <Code>false</Code> on every line in the log table, based on
+          whether the regex matches that line's text case-insensitively. Unlike <Code>string</Code>/<Code>number</Code>
+          /<Code>timestamp</Code>/<Code>duration</Code> params, a <Code>boolean</Code> param is never omitted for a
+          non-matching line.
+        </p>
       </div>
     ),
   },