[skip-ci] rescue collapsed-by-default section of TFile details

See related discussion: root-project#14929

Author: ferdymercury

io/io/src/TFile.cxx

@@ -17,6 +17,7 @@
 \sa \ref IO
 \sa \ref rootio (or `io/doc/TFile` folder in your codebase)
 
+<details>
 <summary>ROOT file data format specification</summary>
 
 A ROOT file is composed of a header, followed by consecutive data records
@@ -77,6 +78,7 @@ Begin_Macro
 End_Macro
 
 The structure of a directory is shown in TDirectoryFile::TDirectoryFile
+</details>
 */
 
 #include <ROOT/RConfig.hxx>