Remove outdated fast field documentation

[GodTamIt]

Closes #2139

[codecov-commenter]

Codecov Report

Merging #2145 (90ac352) into main (47b315f) will decrease coverage by 0.01%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main    #2145      +/-   ##
==========================================
- Coverage   94.32%   94.32%   -0.01%     
==========================================
  Files         320      320              
  Lines       62032    62032              
==========================================
- Hits        58514    58513       -1     
- Misses       3518     3519       +1     

Files Changed	Coverage Δ
src/schema/bytes_options.rs	100.00% <ø> (ø)
src/schema/date_time_options.rs	100.00% <ø> (ø)
src/schema/ip_options.rs	89.04% <ø> (ø)
src/schema/numeric_options.rs	97.96% <ø> (ø)

... and 1 file with indirect coverage changes

[PSeitz]

Thanks!