Docs: Describe time units supported by DATEDIF function

[sequba]

Context

Improve description of DATEDIF in the built-in-functions guide. Describe the time units.

Types of changes

• Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Related issues:

1. Fixes no description about 'Units' from "DATEDIF(Date1, Date2, Units)" #1462

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement.
• My change is compliant with the OpenDocument standard.
• My change is compatible with Microsoft Excel.
• My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• My changes require a documentation update.
• My changes require a migration guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.22%. Comparing base (83b681e) to head (d331980).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1556   +/-   ##
========================================
  Coverage    97.22%   97.22%           
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  Files          169      169           
  Lines        14526    14526           
  Branches      3171     3171           
========================================
  Hits         14123    14123           
  Misses         396      396           
  Partials         7        7           

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[github-actions]

Performance comparison of head (d331980) vs base (83b681e)

                                     testName |   base |   head | change
------------------------------------------------------------------------
                                      Sheet A | 507.46 |  508.3 | +0.17%
                                      Sheet B | 168.14 | 168.13 | -0.01%
                                      Sheet T | 148.22 | 150.44 | +1.50%
                                Column ranges | 506.07 | 508.63 | +0.51%
Sheet A:  change value, add/remove row/column |  14.74 |  14.37 | -2.51%
 Sheet B: change value, add/remove row/column | 140.01 | 130.61 | -6.71%
                   Column ranges - add column | 142.45 |  136.6 | -4.11%
                Column ranges - without batch | 422.21 | 429.22 | +1.66%
                        Column ranges - batch | 113.84 | 106.52 | -6.43%