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_test.go

@@ -13,7 +13,7 @@ func ExampleChars() {
 }
 
 func ExampleExceptChars() {
-	testExceptChars := Chars("0123456789abcdef")
+	testExceptChars := ExceptChars("0123456789abcdef")
 	fmt.Println(testExceptChars("invalid input"))
 	// Output: contains disallowed characters
 }