Update Google Batch docs to clarify spot retry behaviour

[adamrtalbot]

I found the Google Batch spot documentation a bit hard to follow, I've updated the language here to see if it makes more sense.

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[bentsherman]

Actually @pditommaso it looks like we aren't even using the auto-retry exit codes anymore. I think they were used here:

nextflow/plugins/nf-google/src/main/nextflow/cloud/google/batch/GoogleBatchTaskHandler.groovy

Lines 542 to 545 in cc0163a

	final error = lastEvent?.description
	if( error && (EXIT_CODE_REGEX.matcher(error).find() || BATCH_ERROR_REGEX.matcher(error).find()) ) {
	return new ProcessException(error)
	}

But now there is just a regex matching all 500xx exit codes