Update notes on ddp_spawn accelerator in multi-gpu docs

[awaelchli]

What does this PR do?

Fixes #5358

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[codecov]

Codecov Report

Merging #5402 (899f75f) into master (a9377e3) will decrease coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #5402    +/-   ##
=======================================
- Coverage      93%     93%    -0%     
=======================================
  Files         135     135            
  Lines       10015    9875   -140     
=======================================
- Hits         9338    9185   -153     
- Misses        677     690    +13