[ENH/DOC] Support multi-column inputing, `change_type`

[Zeroto521]

PR Description

Please describe the changes proposed in the pull request:

col could receive a single column or multi-column. But currently, the documentation of change_type only shows supporting a single column.

df[col] = df[col].astype(dtype)

PR Checklist

Please ensure that you have done the following:

1. [x] PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

2. [x] If you're not on the contributors list, add yourself to AUTHORS.md.

3. [x] Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Automatic checks

There will be automatic checks run on the PR. These include:

• Building a preview of the docs on Netlify
• Automatically linting the code
• Making sure the code is documented
• Making sure that all tests are passed
• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[codecov[bot]]

Codecov Report

Merging #1163 (3d76102) into dev (3279b06) will increase coverage by 0.30%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1163      +/-   ##
==========================================
+ Coverage   97.59%   97.89%   +0.30%     
==========================================
  Files          78       76       -2     
  Lines        3538     3380     -158     
==========================================
- Hits         3453     3309     -144     
+ Misses         85       71      -14     

[ericmjl]

When all tests pass, I will hit the merge button :smile:.