Clarify SymbolInformation.symbol uniqueness properties

Author: mgsloan

scip.proto

@@ -233,6 +233,8 @@ message Descriptor {
 message SymbolInformation {
   // Identifier of this symbol, which can be referenced from `Occurence.symbol`.
   // The string must be formatted according to the grammar in `Symbol`.
+  // This identifier must be unique within indexer output, unless it begins with
+  // `local `, in which case it must be unique within the `Document`.
   string symbol = 1;
   // (optional, but strongly recommended) The markdown-formatted documentation
   // for this symbol. Use `SymbolInformation.signature_documentation` to