[DOCS] Remove _all for <snapshot> parameter

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

docs/reference/snapshot-restore/apis/get-snapshot-api.asciidoc

@@ -67,13 +67,9 @@ cluster, omit this parameter or use `*` or `_all`.
 
 `<snapshot>`::
 (Required, string)
-Comma-separated list of snapshot names to retrieve. Also accepts wildcards (`*`).
+Comma-separated list of snapshot names to retrieve. Also accepts wildcards (`*`) when a repository is specified.
 +
-* To get information about all snapshots in a registered repository, use a wildcard (`*`) or `_all`.
-* To get information about any snapshots that are currently running, use `_current`.
-+
-NOTE: Using `_all` in a request fails if any snapshots are unavailable.
-Set <<get-snapshot-api-ignore-unavailable,`ignore_unavailable`>> to `true` to return only available snapshots.
+To get information about any snapshots that are currently running, use `_current`.
 
 [role="child_attributes"]
 [[get-snapshot-api-query-params]]
@@ -459,4 +455,4 @@ The API returns the following response:
 // TESTRESPONSE[s/"start_time_in_millis": 1593093628850/"start_time_in_millis": $body.snapshots.0.start_time_in_millis/]
 // TESTRESPONSE[s/"end_time": "2020-07-06T21:55:18.129Z"/"end_time": $body.snapshots.0.end_time/]
 // TESTRESPONSE[s/"end_time_in_millis": 1593094752018/"end_time_in_millis": $body.snapshots.0.end_time_in_millis/]
-// TESTRESPONSE[s/"duration_in_millis": 0/"duration_in_millis": $body.snapshots.0.duration_in_millis/]
+// TESTRESPONSE[s/"duration_in_millis": 0/"duration_in_millis": $body.snapshots.0.duration_in_millis/]