Clarify that SymbolInformation.symbol should be unique

Author: mgsloan

scip.proto

@@ -231,7 +231,7 @@ message Descriptor {
 // SymbolInformation defines metadata about a symbol, such as the symbol's
 // docstring or what package it's defined it.
 message SymbolInformation {
-  // Identifier of this symbol, which can be referenced from `Occurence.symbol`.
+  // Unique identifier of this symbol, which can be referenced from `Occurence.symbol`.
   // The string must be formatted according to the grammar in `Symbol`.
   string symbol = 1;
   // (optional, but strongly recommended) The markdown-formatted documentation