Add explanation to the dynamic description task

infra/GutUsageExamples.md

@@ -147,8 +147,10 @@ gut make -o giellalt -r "(lang-|giella-)" private
 
 # Task 11: Description w/ dynamic content
 
+Use a script to generate the content, including dynamic parts that varies with the repo name, and use the script as follows:
+
 ```sh
-gut set info -o giellalt -r 'lang-XXX' --des-script giella-core/devtools/gut-scripts/reponame2description.sh
+gut set info -o giellalt -r '^lang-' --des-script giella-core/devtools/gut-scripts/reponame2description.sh
 ```
 
 **NB!** Make sure there is no trailing newline at the end of the output of the script, or it will fail. That is, use `printf`,  *not* `echo`.