Add comment explaining the purpose of the env variables

src/termio/Exec.zig

@@ -612,6 +612,8 @@ const Subprocess = struct {
             try env.put("COLORTERM", "truecolor");
         }
 
+        // Set environment variables used by some programs (such as neovim) to detect
+        // which terminal emulator and version they're running under.
         try env.put("TERM_PROGRAM", "ghostty");
         try env.put("TERM_PROGRAM_VERSION", build_config.version_string);