Document how to suppress warnings

[trexfeathers]

Needs #5472

Explain to users how they can get Iris' warnings to work for them - only showing the warnings they need, either via Python scripts or command line configuration.

User Guide? Further Topics? Somewhere else?

Some notes from @bjlittle

• Standard warnings filter initialisation through:
  • Python CLI e.g., python –W arg
  • Python environment variable PYTHONWARNINGS
  • Convenient shorthand e.g., -Wi and -Wd
• Use of –W and PYTHONWARNINGS allows user warnings control with no code changes
• Can interpret warnings customisation through sys.warnoptions (unparsed)
• Warnings filter specifications action:message:category:module:line
• There are several types of warnings filter action

[trexfeathers]

Thanks @bjlittle!