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_paramhlp.c

@@ -135,7 +135,7 @@ ParameterError file2memory_range(char **bufp, size_t *size, FILE *file,
         offset = starto;
       }
       else
-        /* we can't seek stdin, read 'starto' bytes and throw them away */
+        /* we cannot seek stdin, read 'starto' bytes and throw them away */
         throwaway = starto;
     }