fix incorrect and vague doc comments

Audit ahead of the README/doc.go work (issue #3):

- number.go: Min, MinFloat32, MinFloat64 all said "less than or equal to a
  given maximum" -- copy-pasted from the Max family. They check the
  minimum. Fixed.
- chars_test.go: ExampleExceptChars called Chars, not ExceptChars, so it
  never exercised the function it documents.
- chars.go: "does not contain disallowed characters" was circular.
- in.go: NotIn's parameter was named "allow"; renamed to "disallow".
- length.go: note that length is bytes (len), not runes -- the "%d
  characters" messages imply otherwise.
- url.go: spell out that ParseRequestURI wants an absolute URL or path.
- uuid.go: note lowercase-hex only, version/variant not checked.
- error.go: explain Err (the match-anything sentinel), how Error() formats
  with Data, and what With does. all.go: point at Collect.

Comments only; no behaviour change. go vet + go test ./... pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

chars.go

@@ -20,7 +20,7 @@ func Chars(allow string) func(string) error {
 	}
 }
 
-// ExceptChars validates whether a string does not contain disallowed characters.
+// ExceptChars validates that a string contains none of the given characters.
 func ExceptChars(disallow string) func(string) error {
 	return func(value string) error {
 		for _, r := range disallow {