doc, alternator: shorten description of "tags" compatibility

The "current compatibility with DynamoDB" section in alternator.md is where
we should list very briefly our state of compatibility - it's not the right
place to explain implementation details or track obscure bugs. I've
significantly shortened the "Tags" section because, in brief, we do
fully support tags and should say that we do.

I moved the two bugs mentioned in the text into the bug tracker:
Refs #6389
Refs #6391

Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20200507125022.22608-1-nyh@scylladb.com>

docs/alternator/alternator.md

@@ -130,19 +130,9 @@ implemented, with the following limitations:
   (change data capture), but the "DynamoDB Streams" API is not yet supported.
 ### Encryption at rest
 * Supported by Scylla Enterprise (not in open-source). Needs to be enabled.
-### ARNs and tags
-* 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.
+### Tags
+* Tagging tables is fully supported, at CreateTable time (with the "Tags"
+  parameter) and later using TagResource, UntagResource and ListTagsOfResource.
 ### 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