doc: use '?' in optional functions (#8583)

doc/docs.md

@@ -3800,17 +3800,19 @@ module global (so that you can use `ls()` instead of `os.ls()`, for example).
 // so it can be run just by specifying the path to the file
 // once it's made executable using `chmod +x`.
 
-rm('build/*')
+rm('build/*')?
 // Same as:
-for file in ls('build/') {
-    rm(file)
+files_build := ls('build/')?
+for file in files_build {
+    rm(file)?
 }
 
-mv('*.v', 'build/')
+mv('*.v', 'build/')?
 // Same as:
-for file in ls('.') {
+files := ls('.')?
+for file in files {
     if file.ends_with('.v') {
-        mv(file, 'build/')
+        mv(file, 'build/')?
     }
 }
 ```