[docs] Update PostingList string representation format

[kbobyrev]

Summary

Because PostingList objects are compressed, it is now impossible to see elements other than the current one and the documentation doesn't match implementation anymore.

[ioeric]

clang-tools-extra/clangd/index/dex/Iterator.h

97

nit: [(...)? (IDN | END) (...)?] is accurate but can be really confusing... I think it's okay to be a bit inaccurate with just [ ... CurID ... ].

[kbobyrev]

Simplify the documentation format.

clang-tools-extra/clangd/index/dex/Iterator.h

public:
 
   /// Prints a convenient human-readable iterator representation by recursively
   /// dumping iterators in the following format:
   ///
   /// (Type Child1 Child2 ...)
   ///
   /// Where Type is the iterator type representation: "&" for And, "|" for Or,
   /// ChildN is N-th iterator child. Raw iterators over PostingList are
-  /// represented as "[ID1, ID2, ..., {IDN}, ... END]" where IDN is N-th
-  /// PostingList entry and the element which is pointed to by the PostingList
-  /// iterator is enclosed in {} braces.
+  /// represented as "[(...)? (IDN | END) (...)?]" where IDN is N-th
+  /// PostingList entry.
   friend llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
                                        const Iterator &Iterator) {
     return Iterator.dump(OS);
   }
 
   constexpr static float DEFAULT_BOOST_SCORE = 1;
 
 private: