This extension allows for defining metadata annotations in
YANG modules. The 'md:annotation' statement can appear only at
the top level of a YANG module or submodule, i.e. it becomes a
new alternative in the ABNF production rule for 'body-stmts'
(sec. 14 in RFC 6020bis).
The argument of the 'md:annotation' statement defines the name
of the annotation. Syntactically it is a YANG identifier as
defined in RFC 6020bis, sec. 6.2.
An annotation defined with this extension statement inherits
the namespace and other context from the YANG module in which
it is defined.
Data type of the annotation value is specified in the same way
as for a leaf data node using the 'type' statement.
Semantics of the annotation and other documentation can be
specified using the following standard YANG substatements (all
are optional): 'description', 'if-feature', 'reference',
'status', and 'units'.
A server announces support for a particular annotation by
including the module in which the annotation is defined among
the advertised YANG modules (e.g., in NETCONF hello message or
yang-library). The annotation then can be attached to any
instance of data node defined in any YANG module that is
advertised by the server.
XML and JSON encoding of annotations is defined in
RFC XXXX.