Add (media/child) form improvements 2

[rosiel]

Replaces #890

GitHub Issue: Slack conversation with C.Z.:

"How can I add my own media types to add media? If I add them, they will not show up under add media"

What does this Pull Request do?

Tells you, when on the "add Media" and "add Members" page, that the bundles below are available because they have the magic field configured to this bundle.

What's new?

Help text on the Add Children (/node/{node}/members/add) and Add Media (/node/{node}/media/add) pages.

How should this be tested?

• does the text show up?
• do you have suggestions to make it better?

Note: I tried to get "this bundle" (the bundle of the current node) to show up as the bundle's human readable name, but without loading a bunch of extra classes I could only get it to show the machine name of the bundle. So I didn't. If you think it's worth it, please send code.

Documentation Status

• Does this change existing behaviour that's currently documented? no.
• Does this change require new pages or sections of documentation? no but new screenshots might be nice.
• Who does this need to be documented for? it is documentation.
• Associated documentation pull request(s): ___ or documentation issue ___ see above.

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

[rosiel]

@whikloj thanks for your comment on #890. I agree. I tried to improve the wording somewhat.

Sorry for the mess up of the previous PR.

• I hope the <code> tag renders okay in different admin themes.
• I realized that being "configured to allow this content type" has no effect on whether the media/content type shows up in this list, so I removed it.
• The "Manage media types" link only shows up if you have permission to get to the route. 😄

[nigelgbanks]

Reviewed and tested, presents the links with descriptions as described.