Update iteration-speed.md

Mention the new flag from bazelbuild#16805 that can help prevent analysis cache discards

Closes bazelbuild#19989.

PiperOrigin-RevId: 578673354
Change-Id: I062274f8059bbfeef5fb49b9c6c26db9887efde9

site/en/advanced/performance/iteration-speed.md

@@ -90,3 +90,6 @@ server was restarted. Either of these should be avoided during iterative use:
 *   If you want to use multiple sets of flags from the same workspace, you can
     use multiple, distinct output bases, switched with the `--output_base`
     flag. Each output base gets its own Bazel server.
+
+To make this condition an error rather than a warning, you can use the
+`--noallow_analysis_cache_discard` flag (introduced in Bazel 6.4.0)