docs: Document removal of 64-bit on 32-bit emulation

With acce728 we disallowed configuring 64-bit guests on
32-bit hosts, but forgot to document that in removed-features.

Signed-off-by: Richard Henderson <[email protected]>
Reviewed-by: Michael Tokarev <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>

Author: rth7680

docs/about/removed-features.rst

@@ -858,6 +858,15 @@ QEMU.  Since all recent x86 hardware from the past >10 years is
 capable of the 64-bit x86 extensions, a corresponding 64-bit OS should
 be used instead.
 
+32-bit hosts for 64-bit guests (removed in 10.0)
+''''''''''''''''''''''''''''''''''''''''''''''''
+
+In general, 32-bit hosts cannot support the memory space or atomicity
+requirements of 64-bit guests.  Prior to 10.0, QEMU attempted to
+work around the atomicity issues in system mode by running all vCPUs
+in a single thread context; in user mode atomicity was simply broken.
+From 10.0, QEMU has disabled configuration of 64-bit guests on 32-bit hosts.
+
 Guest Emulator ISAs
 -------------------