curldown: fixups

- make DEFAULT sections less repetitive

- make historic mentions use HISTORY

- generate the protocols section on `# %PROTOCOLS%` instead of guessing
  where to put it

- generate the availability section on `# %AVAILABILITY%` instead of
  guessing where to put it

- make the protocols section more verbose

Closes #14227

docs/libcurl/opts/CURLINFO_COOKIELIST.md

@@ -37,6 +37,8 @@ received) the 'struct curl_slist *' is made a NULL pointer.
 Since 7.43.0 cookies that were imported in the Set-Cookie format without a
 domain name are not exported by this option.
 
+# %PROTOCOLS%
+
 # EXAMPLE
 
 ~~~c
@@ -72,6 +74,8 @@ int main(void)
 }
 ~~~
 
+# %AVAILABILITY%
+
 # RETURN VALUE
 
 Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.