docs: mm: fix vm overcommit documentation for OVERCOMMIT_GUESS

Commit 8c7829b04c52 "mm: fix false-positive OVERCOMMIT_GUESS failures"
changed the behavior of the default OVERCOMMIT_GUESS setting.
Reflect the change also in the Documentation, namely files:
Documentation/admin-guide/sysctl/vm.rst
Documentation/mm/overcommit-accounting.rst

Reported-by: Jozef Bacik <[e-mail]>
Signed-off-by: Vratislav Bendel <[e-mail]>
Acked-by: Mike Rapoport <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/admin-guide/sysctl/vm.rst

···
 
 This value contains a flag that enables memory overcommitment.
 
-When this flag is 0, the kernel attempts to estimate the amount
-of free memory left when userspace requests more memory.
+When this flag is 0, the kernel compares the userspace memory request
+size against total memory plus swap and rejects obvious overcommits.
 
 When this flag is 1, the kernel pretends there is always enough
 memory until it actually runs out.

Documentation/mm/overcommit-accounting.rst

···
 	Heuristic overcommit handling. Obvious overcommits of address
 	space are refused. Used for a typical system. It ensures a
 	seriously wild allocation fails while allowing overcommit to
-	reduce swap usage.  root is allowed to allocate slightly more
-	memory in this mode. This is the default.
+	reduce swap usage. This is the default.
 
 1
 	Always overcommit. Appropriate for some scientific