fix: avoid check if email is overridden

[shreyanshdwivedi]

Fixes #6298
Follow-up PR for fossasia/open-event-frontend#3352

Short description of what this resolves:

When email is overridden, a check is still being called and is-email-overridden missing in docs

Changes proposed in this pull request:

• fix check
• add is-email-overridden to docs

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[shreyanshdwivedi]

@iamareebjamal please review

[codecov]

Codecov Report

Merging #6299 into development will not change coverage.
The diff coverage is 0%.

@@             Coverage Diff              @@
##           development    #6299   +/-   ##
============================================
  Coverage        65.45%   65.45%           
============================================
  Files              287      287           
  Lines            14661    14661           
============================================
  Hits              9596     9596           
  Misses            5065     5065

Impacted Files	Coverage Δ
app/api/speakers.py	47.87% <0%> (ø)	⬆️

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