chore(slug): add comments to clarify removed unicode blocks

Author: TomJGooding

src/textual/_slug.py

@@ -34,10 +34,10 @@
     r"\U000024C2-\U0001F251"
     r"\U00002702-\U000027B0"
     r"\U0001F1E0-\U0001F1FF"
-    r"\U0001F300-\U0001F5FF"
-    r"\U0001F600-\U0001F64F"
-    r"\U0001F680-\U0001F6FF"
-    r"\U0001F900-\U0001f9FF"
+    r"\U0001F300-\U0001F5FF"  # Miscellaneous Symbols And Pictographs
+    r"\U0001F600-\U0001F64F"  # Emoticons
+    r"\U0001F680-\U0001F6FF"  # Transport and Map Symbols
+    r"\U0001F900-\U0001F9FF"  # Supplemental Symbols and Pictographs
     r"\u200D"
     r"\u2640-\u2642"
 )