[Frontend] NFC: Remove an outdated comment

`performParseOnly` has been able to handle
multiple input files for a while now.

Author: hamishknight

include/swift/Frontend/Frontend.h

@@ -607,14 +607,13 @@ class CompilerInstance {
   void performSema();
 
   /// Parses the input file but does no type-checking or module imports.
-  /// Note that this only supports parsing an invocation with a single file.
   void performParseOnly(bool EvaluateConditionals = false,
                         bool CanDelayBodies = true);
 
   /// Parses and performs name binding on all input files.
   ///
-  /// Like a parse-only invocation, a single file is required. Unlike a
-  /// parse-only invocation, module imports will be processed.
+  /// This is similar to a parse-only invocation, but module imports will also
+  /// be processed.
   void performParseAndResolveImportsOnly();
 
   /// Performs mandatory, diagnostic, and optimization passes over the SIL.