Improved docs for bitmap

[jorgecarleitao]

No backward incompatible changes, just docs easier to follow

[codecov]

Codecov Report

Merging #1022 (c079528) into main (e976c1e) will decrease coverage by 0.00%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##             main    #1022      +/-   ##
==========================================
- Coverage   71.63%   71.62%   -0.01%     
==========================================
  Files         359      359              
  Lines       19904    19915      +11     
==========================================
+ Hits        14259    14265       +6     
- Misses       5645     5650       +5     

Impacted Files	Coverage Δ
src/types/bit_chunk.rs	100.00% <ø> (ø)
src/bitmap/immutable.rs	82.79% <73.52%> (-5.01%)	⬇️
src/bitmap/mutable.rs	90.25% <100.00%> (ø)
src/bitmap/utils/mod.rs	100.00% <100.00%> (ø)
src/io/ipc/read/read_basic.rs	67.64% <100.00%> (ø)
src/bitmap/utils/slice_iterator.rs	87.93% <0.00%> (+1.72%)	⬆️

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