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>

length.go

@@ -5,7 +5,8 @@ var (
 	ErrMustBeShorter = NewError("must contain no more than %d characters")
 )
 
-// MaxLength validates the length of a string as being less than or equal to a given maximum.
+// MaxLength validates that a string is no longer than a given maximum.
+// Length is measured in bytes (len), not runes.
 func MaxLength(l int) func(string) error {
 	return func(value string) error {
 		if len(value) > l {
@@ -15,7 +16,8 @@ func MaxLength(l int) func(string) error {
 	}
 }
 
-// MinLength validates the length of a string as being greater than or equal to a given minimum.
+// MinLength validates that a string is at least a given minimum length.
+// Length is measured in bytes (len), not runes.
 func MinLength(l int) func(string) error {
 	return func(value string) error {
 		if len(value) < l {