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/curl_multi_socket.md

@@ -57,6 +57,8 @@ callback is far better than relying on polling the timeout value.
 Usage of curl_multi_socket(3) is deprecated, whereas the function is
 equivalent to curl_multi_socket_action(3) with **ev_bitmask** set to 0.
 
+# %PROTOCOLS%
+
 # EXAMPLE
 
 ~~~c
@@ -76,6 +78,8 @@ int main(void)
 
 curl_multi_socket(3) is deprecated, use curl_multi_socket_action(3) instead!
 
+# %AVAILABILITY%
+
 # RETURN VALUE
 
 CURLMcode type, general libcurl multi interface error code.