docs: asyncify

[SimonSchick]

#9552 # Pull Request check-list

Please make sure to review and check all of these items:

• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Have you added new tests to prevent regressions?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Did you update the typescript typings accordingly (if applicable)?
• Did you follow the commit message conventions explained in CONTRIBUTING.md?

Description of change

Update js docs and ts to use async await.
Improved describeTable typings while at it.

[codecov]

Codecov Report

Merging #12297 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12297   +/-   ##
=======================================
  Coverage   96.36%   96.36%           
=======================================
  Files          95       95           
  Lines        9125     9125           
=======================================
  Hits         8793     8793           
  Misses        332      332           

Impacted Files	Coverage Δ
lib/associations/belongs-to-many.js	98.02% <ø> (ø)
lib/model.js	96.63% <ø> (ø)
lib/sequelize.js	95.81% <ø> (ø)
lib/transaction.js	100.00% <ø> (ø)

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[sushantdhiman]

LGTM apart from changes suggested by @papb