docs: add JSDoc

[stdlib-bot]

This PR updates TypeScript declarations for the @stdlib/fft/base/fftpack/ndarray/float64 namespace.

Reviewer Checklist

• Check the scope of the changes (following Conventional Commits):
  • Are these new APIs? Then this is a feat.
  • Are these changes to existing APIs that could break compatibility? Then this is a feat! (i.e., a breaking change).
  • Are these only documentation changes to existing APIs? Then this is docs.
• Update the PR title to align with the change type (feat, feat!, or docs).
• Approve the PR once you are confident about the classification and changes made.

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
fft/base/fftpack/ndarray/float64	$\\color{red}185/194$ $\\color{green}+95.36\\%$	$\\color{green}3/3$ $\\color{green}+100.00\\%$	$\\color{red}0/1$ $\\color{green}+0.00\\%$	$\\color{red}185/194$ $\\color{green}+95.36\\%$

The above coverage report was generated for the changes in this PR.