fix: update docs for getAccessTokenByClientCredentials, other docs fixes

[shazron]

Motivation and Context

The new OAuth Server to Server credentials grant type call returns an expiry in seconds, not milliseconds, like the older API. This will clarify things in the docs.

Also, other doc warnings have been fixed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #139 (8879e2e) into master (8d2d62a) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #139   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          488       488           
  Branches        68        68           
=========================================
  Hits           488       488           

Files Changed	Coverage Δ
src/ValidationCache.js	100.00% <ø> (ø)
src/ims.js	100.00% <ø> (ø)

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