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>

error.go

@@ -2,10 +2,9 @@ package validate
 
 import "fmt"
 
-// Validation error.
-var (
-	Err = Error{}
-)
+// Err is the zero Error. It carries no message, so errors.Is(x, Err) is
+// true for any error produced by this package.
+var Err = Error{}
 
 // Error represents a validation error.
 type Error struct {
@@ -13,8 +12,9 @@ type Error struct {
 	Data    []any
 }
 
-// Error retrieves the message of a validation Error.
-// If it has Data, the message will be formatted.
+// Error returns the error message. If Data is non-empty, Message is used
+// as an fmt.Sprintf format string and Data as its arguments (this is how
+// the sentinels with %d/%q verbs are filled in, e.g. via With).
 func (e Error) Error() string {
 	if len(e.Data) > 0 {
 		return fmt.Sprintf(e.Message, e.Data...)
@@ -34,6 +34,10 @@ func (e Error) Is(target error) bool {
 	return false
 }
 
+// With returns a copy of the Error with value appended to Data, so it
+// lands in the message when Message contains a formatting verb:
+//
+//	ErrMustBeLonger.With(4) // "must contain at least 4 characters"
 func (e Error) With(value any) Error {
 	if e.Data == nil {
 		e.Data = []any{}