doc: Mention minimum Linux-libre version for containers.

* doc/guix.texi ("invoking guix environment", "invoking guix system"):
  Mention Linux-libre requirements for container features.

doc/guix.texi

@@ -4725,6 +4725,10 @@ working directory are mounted:
 guix environment --ad-hoc --container guile -- guile
 @end example
 
+@quotation Note
+The @code{--container} option requires Linux-libre 3.10 or newer.
+@end quotation
+
 The available options are summarized below.
 
 @table @code
@@ -7370,6 +7374,10 @@ guix system container my-config.scm \
    --expose=$HOME --share=$HOME/tmp=/exchange
 @end example
 
+@quotation Note
+This option requires Linux-libre 3.10 or newer.
+@end quotation
+
 @end table
 
 @var{options} can contain any of the common build options provided by