Misc doc updates

Starting to do a complete review of all the Relish docs. I often find myself using these daily or trying to point users to them. I've created a list of things that need updating / adding. This is the first pass on the core docs fixing mostly minor issues.

- [x] Capitalize the first word of scenarios
- [x] Use back ticks for `inline code` and `options` consistently
- [x] Minor grammatical issues
- [x] Fix whitespace wraps as viewed on Relish (wrap to a general 80 cols)
- [x] Replace `describe` instances with `RSpec.describe`
- [x] Clean up any remaining instances of `should` syntax in example specs