Edit pass on System.CommandLine

[gewarren]

Fixes #47692.

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Internal previews

📄 File	🔗 Preview link
docs/standard/commandline/design-guidance.md	Design guidance
docs/standard/commandline/get-started-tutorial.md	Tutorial: Get started with System.CommandLine
docs/standard/commandline/how-to-configure-the-parser.md	How to configure the parser in System.CommandLine
docs/standard/commandline/how-to-customize-help.md	Customize help output
docs/standard/commandline/how-to-customize-parsing-and-validation.md	docs/standard/commandline/how-to-customize-parsing-and-validation
docs/standard/commandline/how-to-enable-tab-completion.md	Tab completion for System.CommandLine
docs/standard/commandline/how-to-parse-and-invoke.md	"How to parse and invoke the result"
docs/standard/commandline/index.md	System.CommandLine overview
docs/standard/commandline/migration-guide-2.0.0-beta5.md	docs/standard/commandline/migration-guide-2.0.0-beta5
docs/standard/commandline/syntax.md	Syntax overview: Commands, options, and arguments

[adamsitnik]

WOW, this is a great improvement. Big thanks for being so detailed and finding all the places that could be improved @gewarren !