Add an explanation of choice of 20k characters

Author: edoardopirovano

Diff for: src/codeql.ts

@@ -901,6 +901,9 @@ export function getExtraOptions(
  *     printing many log lines.
  * (2) It avoids us hitting the limit of how much data we can send in our
  *     status reports on GitHub.com.
+ * This value was chosen because the maximum size of fields in the status
+ * report is 100kb and UTF8 characters can be up to four bytes. 5,000
+ * characters are left over to include other information.
  */
 const maxErrorSize = 20_000;