Rust: Make the QLDoc for fromSource a bit more precise.

rust/ql/lib/codeql/files/FileSystem.qll

@@ -38,7 +38,10 @@ module Folder = Impl::Folder;
 
 /** A file. */
 class File extends Container, Impl::File {
-  /** Holds if this file was extracted from ordinary source code. */
+  /**
+   * Holds if this file was extracted from the source code of the target project
+   * (rather than another location such as inside a dependency).
+   */
   predicate fromSource() {
     exists(ExtractorStep s | s.getAction() = "Extract" and s.getFile() = this)
   }