Document how to invoke tags within tag groups in `/tag` command

[hedyhli]

We don't currently have any tag groups, as far as I know (#2423), but the /tag command docstring references it and it can perhaps be made clearer (or more concisely describe) how tags in tag groups can be invoked.

Proposed by @RohanJnr in #2583:

We need better help message to tell users on how to use tags from a group. Currently, the syntax to view a tag from a group is /tag name:<group_name> <tag_name>, this has to be mentioned in the tag help message.

We could also change the slash command auto complete to show tags under groups and invoke them by just selecting it from the suggested items. Currently, only tag names outside groups are shown