add further info about solution file extensions

Author: TerryGLee

docs/ide/solutions-and-projects-in-visual-studio.md

@@ -45,7 +45,7 @@ Project files are based on the [MSBuild XML schema](../msbuild/msbuild-project-f
 
 ## Solutions
 
-A project is contained within a *solution*. Despite its name, a solution is not an "answer". It's simply a container for one or more related projects, along with build information, Visual Studio window settings, and any miscellaneous files that aren't associated with a particular project. A solution is described by a text file (extension *.sln*) with its own unique format; it's not intended to be edited by hand.
+A project is contained within a *solution*. Despite its name, a solution is not an "answer". It's simply a container for one or more related projects, along with build information, Visual Studio window settings, and any miscellaneous files that aren't associated with a particular project.
 
 ### Solution file
 
@@ -56,6 +56,9 @@ Visual Studio uses two file types (*.sln* and *.suo*) to store settings for solu
 |.sln|Visual Studio Solution|Organizes projects, project items, and solution items in the solution.|
 |.suo|Solution User Options|Stores user-level settings and customizations, such as breakpoints.|
 
+> [!IMPORTANT]
+> A solution is described by a text file (extension *.sln*) with its own unique format; it's not intended to be edited by hand. Conversely, the *.suo* file is a hidden file that is not displayed under the default File Explorer settings. To show hidden files, on the **View** menu in File Explorer, select the **Hidden Items** checkbox.
+
 ### Solution folder
 
 A "solution folder" is a virtual folder that's only in **Solution Explorer**, where you can use it to group projects in a solution. If you want to locate a solution file on a computer, go to **Tools** > **Options** > **Projects and Solutions** > **Locations**. For more information, see [Options dialog box: Projects and Solutions > Locations](./reference/projects-solutions-locations-options.md).