DS-4524 add comment about the use of solr

[abollini]

As discussed in https://github.com/DSpace/DSpace/pull/2707#discussion_r395141579

[benbosman]

Isn't this feedback primarily relevant for POST /api/core/communities/<:uuid>/adminGroup as it's during the creating of the new admin group that a re-index is required to ensure the admin has implicit read rights to all content in the community

[tdonohue]

Rather than adding this as a note, is there a way to simply automate reindexing a Collection/Community when an Admin group is added? This note seems highly unlikely to ever be seen by end users (who would create an Community/Collection Admin group using the UI, and would be unaware that this task requires a reindex).

So, to me, it seems like we either should automate this reindex (when such a group is created) or perhaps make this reindexing of Community/Collection easy to perform from the UI (afterall, we do have the index-discovery script enabled via the /api/system/scripts endpoint).