Add getters for checker properties

Also use `documentation-symbol` to extract documentation from checkers
in a standard way.
Refine checker help output
Complete with *all* checkers when reading from minibuf

Also require input to match the completion list to avoid input of
non-checker symbols.
Restore next-error-function when disabling flycheck-mode
Signal an error if we can't navigate further
Use `user-error` appropriately