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

lib/speedcheck.c

@@ -49,7 +49,7 @@ CURLcode Curl_speedcheck(struct Curl_easy *data,
   if((data->progress.current_speed >= 0) && data->set.low_speed_time) {
     if(data->progress.current_speed < data->set.low_speed_limit) {
       if(!data->state.keeps_speed.tv_sec)
-        /* under the limit at this very moment */
+        /* under the limit at this moment */
         data->state.keeps_speed = now;
       else {
         /* how long has it been under the limit */