Update throttler config docs for v17

[shlomi-noach]

Updating docs per changes in https://github.com/vitessio/vitess/pull/13130:

• throttler-config-via-topo now defaults true
• per-tablet throttler configuration flags are scheduled to be deprecated

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[mattlord]

Unrelated to the PR, but since we're editing the page, we should also turn the single dashes to double dashes in the code snippet at the end of this section: https://deploy-preview-1489--vitess.netlify.app/docs/17.0/reference/features/tablet-throttler/#v16-v15-and-before

[shlomi-noach]

fixed single->double dashes