staking: Documentation fixes

[andreitrand]

Added several documentation-related fixes. In more detail:

• Typo fixes and rephrasing inside the pallet's Readme file
• Various code comment fixes

Related issue: N/A

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/17673524594
Failed job name: fmt

[sigurpol]

thanks for the PR! Whereas all changes make sense, I would suggest to focus not on staking pallet but on staking-async which is the new kid in town.
Reason being that with AHM we are in the process of migrating staking on Westend (done), Kusama (beginning of October), Polkadot (beginning of November) to AH where we use staking-async.

AFAIK there is (or was and maybe it's on hold due to other priorities) an effort by @kianenigma and @Ank4n to have the new staking async pallet thoroughly documented. Kian can surely tell you more about it

[sigurpol]

All changes make definitely sense. As per #9728 (comment), I think though that we should focus on the documentation of staking-async instead (but it doesn't hurt to have fixes in the legacy one ofc)