docs: add entries for alternator tags and arn

Support for tagging and arn was added already, so the documentation
is properly extended.

docs/alternator/alternator.md

@@ -140,8 +140,18 @@ implemented, with the following limitations:
 ### Encryption at rest
 * Supported natively by Scylla, but needs to be enabled by default.
 ### ARNs and tags
-* Various features use ARN (Amazon Resource Names) which we don't support.
-* Not yet supported: ListTagsOfResource, TagResource, UntagResource.
+* ARN is generated for every alternator table
+* Tagging can be used with the help of the following requests:
+  ListTagsOfResource, TagResource, UntagResource.
+  Tags are stored in a schema table (system\_schema.tables.extensions['tags']),
+  which in particular means that concurrent adding of tags for a single table
+  on more than a single node may result in a race, until Scylla schema agreement
+  is reimplemented to avoid them.
+  Also, during table creation, a 'Tags' parameter can be used
+  and it will be honored by alternator. Note however, that creating a table
+  and tagging it later are not atomic operations, so in case of failure it's possible
+  for first to succeed (and leave side effects in the form of a table) and for the second
+  one to fail, adding no tags to the table.
 ### Accounting and capping
 * Not yet supported. Mainly for multi-tenant cloud use, we need to track
   resource use of individual requests (the API should also optionally