Add functional metric docs [skip ci]

[justusschock]

What does this PR do?

Fixes # (issue)

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[pep8speaks]

Hello @justusschock! Thanks for opening this PR.

• In the file pytorch_lightning/metrics/functional/nlp.py:

Line 27:49: E203 whitespace before ':'

Do see the Hitchhiker's guide to code style

[teddykoker]

Great, Thanks Justus :)

[Borda]

need to merge the latest #3949 to pass the tests

[codecov]

Codecov Report

Merging #3946 into master will decrease coverage by 1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #3946    +/-   ##
=======================================
- Coverage      88%     87%    -1%     
=======================================
  Files         109     109            
  Lines        7827    7764    -63     
=======================================
- Hits         6888    6758   -130     
- Misses        939    1006    +67