docs: Update sync_dist warning for multiple processes

[kaushikb11]

What does this PR do?

sync_fn is not a parameter for self.log.

Related to #6716

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[codecov]

Codecov Report

Merging #6790 (f4e09fb) into master (13f67ad) will decrease coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #6790    +/-   ##
=======================================
- Coverage      92%     87%    -5%     
=======================================
  Files         192     192            
  Lines       12174   12224    +50     
=======================================
- Hits        11147   10614   -533     
- Misses       1027    1610   +583     

[carmocca]

Could instead mention sync_dist_op

https://github.com/PyTorchLightning/pytorch-lightning/blob/7f91c5ebbc60b5ebed61072846da606f5d3957e6/pytorch_lightning/core/lightning.py#L222