curldown: make 'added-in:' a mandatory header field

- generate AVAILABILITY manpage sections automatically - for consistent
  wording

- allows us to double-check against other documumentation (symbols-in-versions
  etc)

- enables proper automation/scripting based on this data

- lots of them were wrong or missing in the manpages

- several of them repeated (sometimes mismatching) backend support info

Add test 1488 to verify "added-in" version numbers against
symbols-in-versions.

Closes #14217

docs/libcurl/curl_multi_waitfds.md

@@ -11,6 +11,7 @@ See-also:
   - curl_multi_fdset (3)
 Protocol:
   - All
+Added-in: 8.8.0
 ---
 
 # NAME
@@ -99,10 +100,6 @@ int main(void)
 }
 ~~~
 
-# AVAILABILITY
-
-Added in 8.8.0
-
 # RETURN VALUE
 
 **CURLMcode** type, general libcurl multi interface error code. See