tested adding comments to first file

bash1.sh

 #!/bin/bash
-
+# input arguments
 oldword=$1
 newword=$2
 extension=$3
-
+# find files that have the desired extension
 files=$(find . -type f -name "*.$extension")
-
+# loop over the fiels and switch the old word with the new word
 for file in $files; do
     sed -i "s/$oldword/$newword/g" "$file"
 done