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>

in.go

@@ -4,7 +4,7 @@ var (
 	ErrValueNotAllowed = NewError("not allowed")
 )
 
-// In validates whether a value is found in a slice of allowed values.
+// In validates that a value equals one of the allowed values.
 func In[T comparable](allow ...T) func(T) error {
 	return func(value T) error {
 		for _, cmp := range allow {
@@ -16,10 +16,10 @@ func In[T comparable](allow ...T) func(T) error {
 	}
 }
 
-// NotIn validates whether a value is not found in a slice of disallowed values.
-func NotIn[T comparable](allow ...T) func(T) error {
+// NotIn validates that a value equals none of the disallowed values.
+func NotIn[T comparable](disallow ...T) func(T) error {
 	return func(value T) error {
-		for _, cmp := range allow {
+		for _, cmp := range disallow {
 			if cmp == value {
 				return ErrValueNotAllowed
 			}