Add filterset_fields lookup expresson to docs

[jvacek]

No description provided.

[codecov-commenter]

Codecov Report

Merging #1460 (67b0743) into main (ab79783) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1460      +/-   ##
==========================================
- Coverage   99.44%   99.43%   -0.01%     
==========================================
  Files          15       15              
  Lines        1270     1249      -21     
==========================================
- Hits         1263     1242      -21     
  Misses          7        7              

Impacted Files	Coverage Δ
django_filters/filters.py	100.00% <0.00%> (ø)
django_filters/filterset.py	100.00% <0.00%> (ø)
django_filters/rest_framework/backends.py	100.00% <0.00%> (ø)
django_filters/rest_framework/filterset.py	100.00% <0.00%> (ø)

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[carltongibson]

Ok, thanks.