DOC: management: add forgotten "show startup-logs"

The keyword was never documented.

Must be backported in all maintained versions.
diff --git a/doc/management.txt b/doc/management.txt
index 4b1bd58..63b20a4 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -3435,6 +3435,10 @@
         - fips
         - base
 
+show startup-logs
+  Dump all messages emitted during the startup of the current haproxy process,
+  each startup-logs buffer is unique to its haproxy worker.
+
 show table
   Dump general information on all known stick-tables. Their name is returned
   (the name of the proxy which holds them), their type (currently zero, always