docs: Fix phrasing and punctuation in doc comments

[aoright]

Summary

• In github/github.go, correct on-premise to on-premises in media types preview comment.
• Add missing trailing periods in Client struct field comments.

[google-cla]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.51%. Comparing base (27edd67) to head (5c40273).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4461      +/-   ##
==========================================
+ Coverage   97.55%   98.51%   +0.96%     
==========================================
  Files         194      194              
  Lines       19884    17603    -2281     
==========================================
- Hits        19398    17342    -2056     
+ Misses        268      261       -7     
+ Partials      218        0     -218     

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[gmlewis]

@aoright - please let us know when you fix the cla/google issue, and we can continue with this PR.