badwords: check indented lines in source code, fix fallouts

- badwords.pl: add `-a` option to check all lines in source code files.
  Before this patch indented lines were skipped (to avoid Markdown code
  fences.)
- GHA/checksrc: use `-a` when verifying the source code.
- GHA/checksrc: disable `So` and `But` rules for source code.
- GHA/checksrc: add docs/examples to the verified sources.
- badwords.txt: delete 4 duplicates.
- badwords.txt: group and sort contractions.
- badwords.txt: allow ` url = `, `DIR`, `<file name`.

Closes #19536

src/tool_doswin.c

@@ -874,7 +874,7 @@ curl_socket_t win32_stdin_read_thread(void)
       break;
     }
 
-    /* Start up the thread. We don't bother keeping a reference to it
+    /* Start up the thread. We do not bother keeping a reference to it
        because it runs until program termination. From here on out all reads
        from the stdin handle or file descriptor 0 will be reading from the
        socket that is fed by the thread. */