Add explanation of how to export your key to "Update my key (I edited it elsewhere)"

The "Update my key (I edited it elsewhere)" button doesn't give you any explanation of how to export your key, like the site does the first time you add a key. The user is likely to have forgotten by now; re-explain how to export it here.

Start by comparing the existing first-time key-add explanation with the “Update my key (I edited it elsewhere)” flow named in the issue. Add equivalent export guidance to the update flow, then verify that users can see the instructions when updating an edited key.

Written by the indexing model from the issue text.