Adding documentation about remote mapdl pool

[germa89]

As the title.

Now if we are providing a list of ports, the number of instances argument (n_instances) is optional, not needed.

[ansys-reviewer-bot[bot]]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.13%. Comparing base (5442a4e) to head (81d8020). Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3046      +/-   ##
==========================================
- Coverage   86.63%   84.13%   -2.51%     
==========================================
  Files          52       52              
  Lines        9498     9500       +2     
==========================================
- Hits         8229     7993     -236     
- Misses       1269     1507     +238     

[germa89]

@pyansys-ci-bot LGTM.