Improve multiprocessing documentation

[Sinclert]

The current multiprocessing documentation is short, and can lead to some confusion.

This PR addresses the following aspects:

• Setting up prometheus_multiproc_dir env. var but still fail to collect metrics, usually due to a wrong setting of the variable (see issues Multiprocessing collector doesn't work #490 & Question: how to instrument a multiprocessing program? #382).

• Separating mandatory steps (1 and 2) from optional steps (3), depending on the usage of Gauge. This is because the mark_process_dead function does nothing related to Count, Summary or Histogram metrics, as seen in its definition.

cc @brian-brazil

[Sinclert]

Suggestions applied ✅

[Sinclert]

Suggestions v2 applied ✅

Let me know if there is anything else you want to change. cc/ @brian-brazil

[brian-brazil]

Thanks!