Fixes TRJReader random frame access docs

[IAlibay]

Fixes #2398

Changes made in this Pull Request:

• Removes outdated TRJReader documentation on random frame access

I was going to include this as part of larger PR, but I keep stalling on it, so here's a micro-commit 😅 (sorry)

PR Checklist

• Tests? (random access was already tested for)
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[lilyminium]

LGTM, thanks for doing this @IAlibay!

[codecov]

Codecov Report

Merging #2595 into develop will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #2595   +/-   ##
========================================
  Coverage    90.57%   90.57%           
========================================
  Files          173      173           
  Lines        23383    23383           
  Branches      3038     3038           
========================================
  Hits         21180    21180           
  Misses        1585     1585           
  Partials       618      618           

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/TRJ.py	95.03% <ø> (ø)

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