Add explanatory comment for '.unparsed'

As I don't feel it is completely obvious.

breathe/filetypes.py

@@ -18,4 +18,7 @@ def get_extension(filename: str) -> str:
     # If the filename is just '.ext' then we get ('.ext', '') so we fall back to first part if
     # the second isn't there
     (first, second) = os.path.splitext(filename)
+
+    # Doxygen allows users to specify the file extension ".unparsed" to disable syntax highlighting.
+    # We translate it into the pygments unhighlighted 'text' type
     return (second or first).lstrip(".").replace("unparsed", "text")