main loop: improve defaults, docs and error handling

[oliver-sanders]

• Closes auto restart plugin issues #4873
  (in combination with auto restart: add note about enabling plugin cylc-doc#475)
• There is now a default main loop plugin interval of PT10M.
  This prevents any plugins without a hardcoded default from being run with every main loop cycle.
• Explicitly set a default for the auto restart plugin (also PT10M).
• Added a note on enabling plugins.
• Suppress traceback from reloading the global config in the auto restart plugin.
  This is now logged nicely.
• Prevent plugins from being loaded twice when specified both on the CLI
  (--main-loop) and in the config (global.cylc[scheduler][main loop]plugins).

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Appropriate tests are included (unit and/or functional).
• No change log entry required (why? docs and minor improvements).
• No documentation update required.

[hjoliver]

(Some minor comment and doc suggestions).

[datamel]

Code read, docs built and checked. Checked tests locally too. All good!