doc: Highlight that --distro has to be used with --release

...unless the selected distro matches the host system.

containers#977

Author: martymichal

doc/toolbox-create.1.md

@@ -82,7 +82,8 @@ confusion.
 **--distro** DISTRO, **-d** DISTRO
 
 Create a toolbox container for a different operating system DISTRO than the
-host. Cannot be used with `--image`.
+host. Cannot be used with `--image`. Has to be coupled with `--release` unless
+the selected system matches the host system.
 
 **--image** NAME, **-i** NAME
 

doc/toolbox-enter.1.md

@@ -32,7 +32,8 @@ The following options are understood:
 **--distro** DISTRO, **-d** DISTRO
 
 Enter a toolbox container for a different operating system DISTRO than the
-host.
+host. Has to be coupled with `--release` unless the selected system matches the
+host system.
 
 **--release** RELEASE, **-r** RELEASE
 

doc/toolbox-run.1.md

@@ -34,7 +34,8 @@ entirely customized containers created from custom-built images.
 **--distro** DISTRO, **-d** DISTRO
 
 Run command inside a toolbox container for a different operating system DISTRO
-than the host.
+than the host. Has to be coupled with `--release` unless the selected system
+matches the host system.
 
 **--release** RELEASE, **-r** RELEASE