update docs to refer to server usage limitations

[gboeing]

resolves #1079

[matkoniecz]

If you need to exceed these limitations, consider installing your own hosted instance and setting OSMnx to use it.

Maybe mention that you can also download entire OSM dataset or other major parts? See https://wiki.openstreetmap.org/wiki/Downloading_data

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.83%. Comparing base (68a8ee3) to head (686baf0).
Report is 687 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1080   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files          27       27           
  Lines        2497     2497           
=======================================
  Hits         2443     2443           
  Misses         54       54           

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