meta: update issue templates

.github/ISSUE_TEMPLATE/bug_report.md

 ---
 name: Bug report
 about: Create a report to help us improve
-title: ''
 labels: 'Type: bug'
-assignees: ''
 
 ---
 
+<!--
+Fill this out before posting. You can delete irrelevant sections, but
+an issue where no sections have been filled will be deleted without comment.
+-->
+
 **Describe the bug**
-A clear and concise description of what the bug is.
+<!-- A clear and concise description of what the bug is. -->
 
 **To Reproduce**
+<!--
 Steps to reproduce the behavior:
 1. Enable plugin '...'
 2. Run command '...', _or_ try to complete command '...', _etc._
 3. See error
+-->
 
 **Expected behavior**
-A clear and concise description of what you expected to happen.
+<!-- A clear and concise description of what you expected to happen. -->
 
 **Screenshots or recordings**
-If applicable, add screenshots or record an [asciinema session](https://asciinema.org/) to help explain your problem.
+<!--
+If applicable, add screenshots or record an asciinema session (https://asciinema.org/)
+to help explain your problem.
+-->
 
-**Desktop (please complete the following information):**
+**System:**
  - OS: [e.g. macOS]
  - Zsh version [e.g. 5.6]
  - Terminal emulator [e.g. iTerm2]
 
 **Additional context**
-Add any other context about the problem here.
+<!-- Add any other context about the problem here. -->

.github/ISSUE_TEMPLATE/feature_request.md

 ---
 name: Feature request
 about: Suggest an idea for Oh My Zsh
-title: ''
 labels: 'Type: feature request'
-assignees: ''
 
 ---
 
+<!--
+Fill this out before posting. You can delete irrelevant sections, but
+an issue where no sections have been filled will be deleted without comment.
+-->
+
 **Is your feature request related to a particular plugin or theme? If so, specify it.**
-The name of the plugin or theme you'd like us to improve.
+<!-- The name of the plugin or theme you'd like us to improve. -->
 
 **Is your feature request related to a problem? Please describe.**
-A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+<!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
 
 **Describe the solution you'd like**
-A clear and concise description of what you want to happen.
+<!-- A clear and concise description of what you want to happen. -->
 
 **Describe alternatives you've considered**
-A clear and concise description of any alternative solutions or features you've considered.
+<!-- A clear and concise description of any alternative solutions or features you've considered. -->
 
 **Additional context**
-Add any other context or screenshots about the feature request here.
+<!-- Add any other context or screenshots about the feature request here. -->

.github/ISSUE_TEMPLATE/support.md

 ---
 name: Support
 about: Request support for any problem you're having with Oh My Zsh
-title: ''
 labels: 'Type: support'
-assignees: ''
 
 ---
 
 <!--
-1. Try to make sure the issue is due to Oh My Zsh
-2. Include as much information as possible
+1. Look for similar issues already posted (including closed ones)
+2. Include as much relevant information as possible
+3. Try to make sure the issue is due to Oh My Zsh
 -->