Update Path.xml (dotnet#5375)

* Update Path.xml

- added additional information about the default path /tmp/  for Linux
- fixes dotnet#5340

* Apply suggestions from code review

Co-authored-by: Adam Sitnik <[email protected]>

Author: Rajakavitha1

xml/System.IO/Path.xml

@@ -1930,7 +1930,7 @@ Paths are resolved by calling the <xref:System.IO.Path.GetFullPath%2A> method be
 
 # [Linux](#tab/linux)
 
-1. The path specified by the TMPDIR environment variable.  
+1. The path specified by the TMPDIR environment variable. If the path is not specified in the `TMPDIR` environment variable, the default path `/tmp/` is used.   
 
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