Improve help messages

[waldyrious]

Various small changes to the help messages to make them more readable and consistent, correct some typos, expand missing information, etc.)

[skywinder]

looks Original is missed?

[waldyrious]

Original is currently not supported as a value that can be passed:

czkawka/czkawka_cli/src/commands.rs

Lines 575 to 585 in 2a32a52

	fn parse_similar_images_similarity(src: &str) -> Result<SimilarityPreset, &'static str> {
	match src.to_lowercase().replace('_', "").as_str() {
	"minimal" => Ok(SimilarityPreset::Minimal),
	"verysmall" => Ok(SimilarityPreset::VerySmall),
	"small" => Ok(SimilarityPreset::Small),
	"medium" => Ok(SimilarityPreset::Medium),
	"high" => Ok(SimilarityPreset::High),
	"veryhigh" => Ok(SimilarityPreset::VeryHigh),
	_ => Err("Couldn't parse the image similarity preset (allowed: Minimal, VerySmall, Small, Medium, High, VeryHigh)"),
	}
	}

I was actually planning to propose adding it as an option (perhaps renaming it to "Maximal" to make it more accurate) in a separate PR :)