_character_to_key(): Fix comment and improve docstring

src/textual/keys.py

@@ -304,15 +304,17 @@ def _get_key_display(key: str) -> str:
 def _character_to_key(character: str) -> str:
     """Convert a single character to a key value.
 
+    If `character` has no key value, return it unchanged.
+
     This transformation can be undone by the function `_get_unicode_name_from_key`.
     """
     if not character.isalnum():
         try:
             key = unicodedata.name(character)
         except (TypeError, ValueError):
-            # ValueError: `character` has no name
-            # TypeError: `character` is not a unicode character (e.g. a control
-            #            character like r"\r")
+            # ValueError: `character` has no name. Control characters, for
+            # example, have no name: https://stackoverflow.com/questions/70074668
+            # TypeError: `character` is not a single character.
             key = character
         else:
             key = key.lower().replace("-", "_").replace(" ", "_")