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

docs/examples/multithread.c

@@ -97,7 +97,7 @@ int main(void)
                            pull_one_url,
                            (void *)&targs[i]);
     if(error)
-      fprintf(stderr, "Couldn't run thread number %d, errno %d\n", i, error);
+      fprintf(stderr, "Could not run thread number %d, errno %d\n", i, error);
     else
       fprintf(stderr, "Thread %d, gets %s\n", i, urls[i]);
   }