[Clang] Fix several `-Wdocumentation` warnings (NFC)

```
clang/include/clang/Lex/Preprocessor.h:2893:14: warning: parameter 'isEnter:' not found in the function declaration [-Wdocumentation]
  /// \param isEnter: true if this PP is entering a region; otherwise, this PP
             ^~~~~~~~

clang/include/clang/Lex/Preprocessor.h:2895:14: warning: parameter 'Loc:' not found in the function declaration [-Wdocumentation]
  /// \param Loc: the location of the entry or exit of a
             ^~~~

clang/include/clang/Lex/Preprocessor.h:2907:14: warning: parameter 'StartLoc:' not found in the function declaration [-Wdocumentation]
  /// \param StartLoc: output argument. It will be set to the start location of
             ^~~~~~~~~
```