Update src/libraries/System.Private.Uri/src/System/UriBuilder.cs

Co-authored-by: Miha Zupan <mihazupan.zupan1@gmail.com>

Author: cdbullard

src/libraries/System.Private.Uri/src/System/UriBuilder.cs

@@ -262,6 +262,9 @@ private static string EncodeUserInfo(string? input)
                 return string.Empty;
             }
 
+            // The following characters ("/" / "\" / "?" / "#" / "@") are from the gen-delims group.
+            // We have to escape them to avoid corrupting the rest of the Uri string.
+            // Other characters like spaces or non-ASCII will be escaped by Uri, we can ignore them here.
             if (input.AsSpan().IndexOfAny(@"/\?#@") < 0)
             {
                 return input;