config-linux: Lift no-tweaking namespace restriction

This restriction originally landed via 02b456e (Clarify behavior
around namespaces paths, 2015-09-08, opencontainers#158).  The hostname case landed
via 66a0543 (config: Require a new UTS namespace for config.json's
hostname, 2015-10-05, opencontainers#214) citing the namespace restriction.  The
restriciton extended to runtime namespaces in 01c2d55 (config-linux:
Extend no-tweak requirement to runtime namespaces, 2016-08-24, opencontainers#538).
There was a proposal in-flight to get config-wide consistency around
the no-tweaking concept [1].

In today's meeting, the maintainer consensus was to strike the
no-tweaking restriction [2], which is what I've done here.

The hostname entry still mentions the UTS namespace to provide a guard
against accidental foot-gunning.  There was no no-tweaking language
for properties related to other namespaces (e.g. 'mounts').
Maybe the other namespaces have more obvious names.

[1]: opencontainers#540
[2]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2017/opencontainers.2017-01-11-22.04.log.html#l-117

Signed-off-by: W. Trevor King <wking@tremily.us>

config-linux.md

@@ -39,7 +39,6 @@ The following parameters can be specified to setup namespaces:
 
 If a path is specified, that particular file is used to join that type of namespace.
 If a namespace type is not specified in the `namespaces` array, the container MUST inherit the [runtime namespace](glossary.md#runtime-namespace) of that type.
-If a new namespace is not created (because the namespace type is not listed, or because it is listed with a `path`), runtimes MUST assume that the setup for that namespace has already been done and error out if the config specifies anything else related to that namespace.
 If a `namespaces` field contains duplicated namespaces with same `type`, the runtime MUST error out.
 
 ###### Example

config.md

@@ -256,7 +256,8 @@ For Windows based systems the user structure has the following fields:
 ## Hostname
 
 * **`hostname`** (string, OPTIONAL) configures the container's hostname as seen by processes running inside the container.
-  On Linux, you can only set this if your bundle creates a new [UTS namespace][uts-namespace].
+  On Linux, this will change the hostname in the [container][container-namespace] [UTS namespace][uts-namespace].
+  Depending on your [namespace configuration](config-linux.md#namespaces), the container UTS namespace may be the [runtime UTS namespace][runtime-namespace].
 
 ### Example