Documentation update, mainly for C API

[jajhall]

Updated highs/interfaces/highs_c_api.h to reflect the need to call Highs_resetGlobalScheduler to free all memory, and updated docs/src/parallel.md to stress that all concurrent instances of Highs must use the same value for the threads option

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.00%. Comparing base (8985537) to head (4d077c2).
⚠️ Report is 78 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3148      +/-   ##
==========================================
- Coverage   73.19%   73.00%   -0.20%     
==========================================
  Files         430      432       +2     
  Lines      104154   104421     +267     
  Branches    16738    16802      +64     
==========================================
- Hits        76239    76236       -3     
- Misses      27639    27909     +270     
  Partials      276      276              

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