[DOCS] Remove _all for <snapshot> parameter

[lockewritesdocs]

Removes the _all option for the <snapshot> parameter of the get snapshot API. The _all option is supported only for the <repository> parameter in 7.13.

Closes #75997

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[DaveCTurner]

I think _all has been legitimate for the {snapshot} parameter since forever, it's the {repository} parameter that has to name a single repository in versions <7.14.

Also NB the docs are right in 7.14, it's versions 7.9--7.13 that are wrong.

[lockewritesdocs]

I think _all has been legitimate for the {snapshot} parameter since forever, it's the {repository} parameter that has to name a single repository in versions <7.14.

I tested on a 7.13.3 cloud instance, and specifying _all for the repository works:

However, when specifying _all for the <snapshot> parameter with a repository, the request fails:

Also NB the docs are right in 7.14, it's versions 7.9--7.13 that are wrong.
@DaveCTurner, this was supposed to be for 7.13 -- I ended up editing the wrong version. I'll create a new PR for 7.13 after we determine the required fix.

[DaveCTurner]

Confusingly GET _snapshot/{repository} is the get-repository API which has supported multiple repositories (and * and _all) since forever. The issue is about the get-snapshots API: GET _snapshot/{repository}/{snapshot}.

I'm surprised by the response to GET _snapshot/my-snapshots/_all, but that doesn't indicate that _all isn't supported, just that the repository is in a weird state.

[lockewritesdocs]

Closing in favor of #76036