Infra: More clearly define, describe and organize exclude patterns

conf.py

@@ -26,28 +26,28 @@
 
 # List of patterns (relative to source dir) to ignore when looking for source files.
 exclude_patterns = [
-    # .txt files in subdirs
+    # Any .txt files in subdirectories:
     "**/*.txt",
-    # .txt files in the root not matching PEP pattern
+    # Non-PEP .txt files in the root (i.e. not "pep-[0-9][0-9][0-9][0-9].txt"):
     "*[!0-9].txt",
     "*[!0-9]?.txt",
     "*[!0-9]??.txt",
     "*[!0-9]???.txt",
     "*[!p]?????.txt",
     "*[!e]??????.txt",
     "*[!p]???????.txt",
-    # Windows:
-    "Thumbs.db",
+    # macOS:
     ".DS_Store",
     # Git
     ".git",
     # Sphinx:
     "build",
-    # PEPs:
-    "pep-0012/*",
+    # Meta files:
     "README.rst",
     "CONTRIBUTING.rst",
     "pep_sphinx_extensions/LICENCE.rst",
+    # Non-built files:
+    "pep-0012/pep-NNNN.rst",
 ]
 
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