Add documentation comments to utf_codepoint

Author: tomwhatmore

src/gleam/utf_codepoint.gleam

@@ -1,20 +1,21 @@
 import gleam/result
 
+/// A UtfCodepoint is the integer representation of a valid UTF codepoint
 pub type UtfCodepoint = UtfCodepoint
 
-pub type Error {
-  Invalid
-}
-
 external fn int_to_utf8_codepoint(Int) -> UtfCodepoint =
   "gleam_stdlib" "identity"
 
-pub fn from_int(value: Int) -> Result(UtfCodepoint, Error) {
+/// Convert an integer to a UtfCodepoint
+///
+/// Returns an error if the integer does not represent a valid UTF codepoint.
+///
+pub fn from_int(value: Int) -> Result(UtfCodepoint, Nil) {
   case value {
-    i if i > 1114111 -> Error(Invalid)
-    i if i == 65534 -> Error(Invalid)
-    i if i == 65535 -> Error(Invalid)
-    i if i >= 55296 && i <= 57343 -> Error(Invalid)
+    i if i > 1114111 -> Error(Nil)
+    i if i == 65534 -> Error(Nil)
+    i if i == 65535 -> Error(Nil)
+    i if i >= 55296 && i <= 57343 -> Error(Nil)
     i -> Ok(int_to_utf8_codepoint(i))
   }
 }