INSTALL: remove unsupported XEN_CONFIG_EXPERT from documentation

Remove XEN_CONFIG_EXPERT explanation and also correct information in
the entire "Xen Hypervisor" section.

Link: 37339ba9ef ("automation: Remove XEN_CONFIG_EXPERT leftovers")
Suggested-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Denis Mukhin <dmukhin@ford.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

INSTALL

@@ -25,19 +25,12 @@ Xen Hypervisor
 Xen itself is configured via a `kconfig' system borrowed from Linux.
 See https://www.kernel.org/doc/html/v5.4/kbuild/.
 
-Note that unlike with Linux, and contrary to that document, you cannot
-look at Kconfig files, or the default or generated config files etc.,
-to find available configuration options.  This is because it is only
-supported (and security supported) by the Xen Project, to change a
-small subset of the options.  Attempts to change other options will be
-silently overridden.  The only way to find which configuration options
-are available is to run `make menuconfig' or the like.
-
-You can counter-override this behaviour by setting XEN_CONFIG_EXPERT=y
-in your environment.  However, doing this is not supported and the
-resulting configurations do not receive security support.  If you set
-this variable there is nothing stopping you setting dangerously
-experimental combinations of features - not even any warnings.
+Only a subset of options is supported or security-supported by Xen
+Project. You can explore all available options, including unsupported
+ones and those recommended only for expert users, e.g. by using `make
+menuconfig` and enabling `CONFIG_UNSUPPORTED` and/or `CONFIG_EXPERT`.
+However, enabling these options is not supported, and configurations
+resulting from them do not receive security support.
 
 Options recognized by configure
 ===============================