docs: Add a note about img tag in media-poster-image.mdx

[ADTC]

This doc change is inspired by the comment by @luwes here:

#904 (comment)

PS: Maintainer edits are allowed. Feel free to change the text as you see fit.

[vercel]

@ADTC is attempting to deploy a commit to the Mux Team on Vercel.

A member of the Team first needs to authorize it.

[luwes]

Thanks for the contribution!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.04%. Comparing base (3ea80df) to head (c441ede).
Report is 97 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #907      +/-   ##
==========================================
+ Coverage   78.55%   79.04%   +0.49%     
==========================================
  Files          59       50       -9     
  Lines       11080    12081    +1001     
  Branches        0      695     +695     
==========================================
+ Hits         8704     9550     +846     
- Misses       2376     2531     +155     

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