Document custom_multichannels

[travishathaway]

Checklist

• I added a descriptive title
• I searched open reports and couldn't find a duplicate

What happened?

We should document custom_multichannels by demonstrating how to use it on this page:

• https://docs.conda.io/projects/conda/en/stable/user-guide/configuration/settings.html

What should be done?

• Start with the current explanation here:

  conda/conda/base/context.py

  Lines 1480 to 1490 in 5b77a1c

  	custom_multichannels=dals(
  	"""
  	A multichannel is a metachannel composed of multiple channels. The two reserved
  	multichannels are 'defaults' and 'local'. The 'defaults' multichannel is
  	customized using the 'default_channels' parameter. The 'local'
  	multichannel is a list of file:// channel locations where conda-build stashes
  	successfully-built packages. Other multichannels can be defined with
  	custom_multichannels, where the key is the multichannel name and the value is
  	a list of channel names and/or channel urls.
  	"""
  	),

  simplifying the language where necessary
• Include an example of how to use it (defaults is a good example)

Additional Context

The lack of its documentation came up on this discussion thread on our discourse server:

• https://conda.discourse.group/t/are-there-any-publicly-available-conda-forge-mirrors/570/6