[Docs] Mention that datamodules can also be used with .test() method

[SkafteNicki]

What does this PR do?

Fixes #5265
Adds to docs that datamodules are also supported as a valid way to pass in an external dataloader

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[codecov]

Codecov Report

Merging #5286 (5bd1b72) into master (dd98a60) will decrease coverage by 1%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5286   +/-   ##
======================================
- Coverage      93%     92%   -1%     
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  Files         134     134           
  Lines        9950   10030   +80     
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+ Hits         9261    9267    +6     
- Misses        689     763   +74     

[rohitgr7]

just minor comments

[s-rog]

Just gotta address rohitgr7's comments!