Update language in "Using OAuth" documentation

Dan left a note here: https://h.readthedocs.io/en/latest/api/using-oauth/#annotations:q_XdmCi6Eeq2T4dVIdSyAg

He points out that we should update the documentation itself to make this clearer to users.

Currently the OAuth documentation says that you should go to the /admin/oauthclients page to register an OAuth client.

After changes we should specify that this is true for Hypothesis admins, and for people who are running their own instance of Hypothesis.

Open the “Using OAuth” documentation at the linked /api/using-oauth/ page and read the note referenced in the issue. Update the statement about /admin/oauthclients so it clearly distinguishes Hypothesis admins from people running their own Hypothesis instance. Done means both audiences are explicitly covered in the documentation.

Written by the indexing model from the issue text.