Update documentation about fjson package

[AlekSi]

Closes #182.

[codecov]

Codecov Report

Merging #262 (4fd04dc) into main (e4e9e24) will not change coverage.
The diff coverage is 60.00%.

@@           Coverage Diff           @@
##             main     #262   +/-   ##
=======================================
  Coverage   51.04%   51.04%           
=======================================
  Files         109      109           
  Lines        4958     4958           
=======================================
  Hits         2531     2531           
  Misses       2029     2029           
  Partials      398      398           

Impacted Files	Coverage Δ
internal/bson/bson.go	0.00% <ø> (ø)
internal/bson/document.go	43.91% <ø> (ø)
internal/clientconn/conn.go	0.00% <ø> (ø)
internal/fjson/fjson.go	85.52% <ø> (ø)
internal/handlers/msg_getcmdlineopts.go	0.00% <0.00%> (ø)
internal/handlers/query_cmd.go	0.00% <0.00%> (ø)
internal/types/array.go	66.10% <ø> (ø)
internal/types/types.go	94.11% <ø> (ø)
internal/wire/op_msg.go	46.00% <ø> (ø)
internal/wire/op_query.go	21.42% <ø> (ø)
... and 3 more

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