Docs: Missing documentation that `['*', 'x', 'X']` can all be wildcards

[nyurik]

By accident, I discovered that VersionReq can parse all 3 of ['*', 'x', 'X'] characters as a wildcard, while it is not documented either here nor in cargo rust docs. Are there any difference between how they are used?

semver/src/parse.rs

Lines 191 to 201 in 4ea60ae

	fn wildcard(input: &str) -> Option<(char, &str)> {
	if let Some(rest) = input.strip_prefix('*') {
	Some(('*', rest))
	} else if let Some(rest) = input.strip_prefix('x') {
	Some(('x', rest))
	} else if let Some(rest) = input.strip_prefix('X') {
	Some(('X', rest))
	} else {
	None
	}
	}

[andrius-puksta-sensmetry]

*, x and X are treated exactly the same. This was added in #247 to match cargo's previous behavior.