docs: netlink: clean up after deprecating version

Jiri moved version to legacy specs in commit 0f07415ebb78 ("netlink:
specs: don't allow version to be specified for genetlink").
Update the documentation.

Reviewed-by: Jiri Pirko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/userspace-api/netlink/genetlink-legacy.rst

···
 Specification
 =============
 
+Globals
+-------
+
+Attributes listed directly at the root level of the spec file.
+
+version
+~~~~~~~
+
+Generic Netlink family version, default is 1.
+
+``version`` has historically been used to introduce family changes
+which may break backwards compatibility. Since compatibility breaking changes
+are generally not allowed ``version`` is very rarely used.
+
 Attribute type nests
 --------------------
 

Documentation/userspace-api/netlink/specs.rst

···
 Name of the family. Name identifies the family in a unique way, since
 the Family IDs are allocated dynamically.
 
-version
-~~~~~~~
-
-Generic Netlink family version, default is 1.
-
 protocol
 ~~~~~~~~