Rust: add missing QLDocs

rust/ql/lib/codeql/rust/Concepts.qll

@@ -290,6 +290,7 @@ module Path {
     }
   }
 
+  /** A data-flow node that checks that a path is safe to access. */
   class SafeAccessCheck extends DataFlow::ExprNode {
     SafeAccessCheck() { this = DataFlow::BarrierGuard<safeAccessCheck/3>::getABarrierNode() }
   }

rust/ql/lib/codeql/rust/frameworks/stdlib/Stdlib.qll

@@ -1,3 +1,7 @@
+/**
+ * Provides classes modeling security-relevant aspects of the standard libraries.
+ */
+
 private import rust
 private import codeql.rust.Concepts
 private import codeql.rust.controlflow.ControlFlowGraph as Cfg

rust/ql/lib/codeql/rust/security/TaintedPathExtensions.qll

@@ -30,6 +30,9 @@ module TaintedPath {
    */
   abstract class Barrier extends DataFlow::Node { }
 
+  /**
+   * A sanitizer guard for path-traversal vulnerabilities.
+   */
   class SanitizerGuard extends DataFlow::Node {
     SanitizerGuard() { this = DataFlow::BarrierGuard<sanitizerGuard/3>::getABarrierNode() }
   }