Update of mmap_mode description

[drfraser]

Update to better describe mmap_mode parameter for Parallel class. Reword as needed...

[codecov]

Codecov Report

Merging #1203 (cd2bc75) into master (0fa2cb9) will decrease coverage by 5.76%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1203      +/-   ##
==========================================
- Coverage   94.51%   88.75%   -5.77%     
==========================================
  Files          47       47              
  Lines        7034     7050      +16     
==========================================
- Hits         6648     6257     -391     
- Misses        386      793     +407     

Impacted Files	Coverage Δ
joblib/parallel.py	96.96% <ø> (+0.55%)	⬆️
joblib/test/test_dask.py	3.73% <0.00%> (-95.15%)	⬇️
joblib/_dask.py	24.13% <0.00%> (-69.96%)	⬇️
joblib/test/test_parallel.py	95.76% <0.00%> (-1.34%)	⬇️
joblib/memory.py	95.37% <0.00%> (-0.19%)	⬇️
joblib/test/test_memory.py	98.62% <0.00%> (+0.01%)	⬆️

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[ogrisel]

Thank you @drfraser!