engine/config: fix wasm_tail_call and wasm_extended_const docs

[airwoodix]

The defaults for Config::tail_call and Config::extended_const were changed in #1031 but the change wasn't reflected in the documentation.

Also fix the markdown reference for the extended-const proposal link.

[Robbepop]

@airwoodix great catch! Thanks for the fix. :) Will merge once CI is green.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.86%. Comparing base (485a30a) to head (3b2d570).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1042   +/-   ##
=======================================
  Coverage   80.86%   80.86%           
=======================================
  Files         271      271           
  Lines       24896    24896           
=======================================
  Hits        20132    20132           
  Misses       4764     4764           

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