[DOCS] Fix get snapshot status API docs

[jrodewig]

As pointed out in #80931, the get snapshot status API docs have some issues:

• GET _snapshot/_status GET _snapshot/<repository>/_status are undocumented.
• The <repository> and <snapshot> parameters are not required.
• The <snapshot> parameter doesn't support _current.

Relates to #59355.

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

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