Fix delete-desired-balance doc (#96978)

This change adds a proper API description instead of one that was
accidentally copied from the other endpoint.

docs/reference/cluster/delete-desired-balance.asciidoc

@@ -1,12 +1,14 @@
 [[delete-desired-balance]]
-=== Delete/reset desired balance API
+=== Reset desired balance API
 ++++
-<titleabbrev>Delete/reset desired balance</titleabbrev>
+<titleabbrev>Reset desired balance</titleabbrev>
 ++++
 
 NOTE: {cloud-only}
 
-Exposes the desired balance and basic metrics.
+Resets the desired balance and starts a new computation from the current allocation.
+This API may be used if desired balance computation diverged from the current state
+and is trying to move too many shards.
 
 [[delete-desired-balance-request]]
 ==== {api-request-title}