[clang-tools-extra] [clangd] Document the cases in which Dex::Files and IdxContents are populated (PR #118906)

[kadir çetinkaya via cfe-commits]

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@@ -121,6 +121,8 @@ class Dex : public SymbolIndex {
   llvm::DenseMap<std::pair<SymbolID, uint8_t>, std::vector<SymbolID>> Relations;
   std::shared_ptr<void> KeepAlive; // poor man's move-only std::any
   // Set of files which were used during this index build.
+  // Files and IdxContents are only populated for dynamic and background
+  // indexes, not static indexes.
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kadircet wrote:

i am not sure if it's worthwhile to encode some state of the users into the library here. they're likely to get out-of-sync quickly, and I don't think they provide any value to the reader. `Dex` is independent of the concepts such as dyanmic/static index.

https://github.com/llvm/llvm-project/pull/118906