update comments

.github/ISSUE_TEMPLATE/bug_report.md

@@ -4,6 +4,7 @@ about: Create a report to help us improve
 labels: bug
 ---
 
+<!-- Please search open and closed issues to avoid duplicates -->
 **Does it happen in OpenBoard, or is it exclusive to this modified version?**
 
 **Describe the bug**

.github/ISSUE_TEMPLATE/feature_request.md

@@ -5,6 +5,7 @@ labels: enhancement
 ---
 
 <!--
+Please search open and closed issues to avoid duplicates.
 Note that this version of OpenBoard is focusing only on a few things. You can add a feature request, but at the current stage it's unlikely it will get implemented.
 -->
 

.github/ISSUE_TEMPLATE/new-language-request.md

@@ -5,6 +5,8 @@ labels: dictionaries
 ---
 
 <!--
+Please search open and closed issues to avoid duplicates.
+
 Before you suggest adding a language, check if the language is already included in the downloadable dictionaries at https://github.com/Helium314/openboard/tree/new/dictionaries/dict or experimental dictionaries at  https://github.com/Helium314/openboard/tree/new/dictionaries/experimental/dict
 
 If you open an issue about an existing dictionary, add information which dictionary you use (included in app, or link)

.github/ISSUE_TEMPLATE/other.md

@@ -4,6 +4,8 @@ about: Anything that does not fit into the other categories
 ---
 
 <!--
+Please search open and closed issues to avoid duplicates.
+
 Note that this version of OpenBoard is focusing only on a few things. You can add a feature request or open other issues, but at the current stage it could take a long time until things are improved or even until you get a response.
 
 If you add screenshots, please reduce the size or use thumbnails to keep the issue nicely readable.