Clarify symbol uniqueness and signature_documentation.relative_path presence

Author: mgsloan

scip.proto

@@ -76,7 +76,8 @@ message Document {
   // This field is typed as a string to permit any programming language, including
   // ones that are not specified by the `Language` enum.
   string language = 4;
-  // (Required) Unique path to the text document.
+  // Unique path to the text document. Required for top-level documents, but
+  // should be empty for `SymbolInformation.signature_documentation` values.
   //
   // 1. The path must be relative to the directory supplied in the associated
   //    `Metadata.project_root`.
@@ -233,6 +234,8 @@ 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