better explain how to disable autolinking for JSON

[grisumbras]

Fix #1036

[cppalliance-bot]

An automated preview of the documentation is available at https://1053.jsondocs.prtest.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1053/pullrequest.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.41%. Comparing base (27c40ee) to head (502ac79).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1053   +/-   ##
========================================
  Coverage    93.41%   93.41%           
========================================
  Files           91       91           
  Lines         8667     8667           
========================================
  Hits          8096     8096           
  Misses         571      571           

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