BZ1926142: Add clarity for machine requirements

In "Minimum resource requirements" table it's mentioned that compute
nodes can use both RHCOS or RHEL 7.9.
This commit fixes the contradiction between the table and the sentence
in the "Required machines" section.

Author: xenolinux

modules/installation-requirements-user-infra.adoc

@@ -99,7 +99,7 @@ these cluster machines.
 endif::ibm-z[]
 ====
 
-The bootstrap and control plane machines must use {op-system-first} as the operating system.
+The bootstrap and control plane machines must use {op-system-first} as the operating system. However, the compute machines can choose between {op-system-first} or {op-system-base-full} 7.9.
 
 ifdef::bare-metal,agnostic[]
 [IMPORTANT]