docs: clarify facebook graph API versioning

[aeneasr]

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.49%. Comparing base (1008639) to head (865c861).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4208      +/-   ##
==========================================
+ Coverage   78.48%   78.49%   +0.01%     
==========================================
  Files         380      380              
  Lines       27143    27143              
==========================================
+ Hits        21302    21306       +4     
+ Misses       4218     4216       -2     
+ Partials     1623     1621       -2     

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