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_waitfds.md

@@ -52,6 +52,8 @@ The client code can pass *size* equal to zero just to get the number of the
 descriptors and allocate appropriate storage for them to be used in a
 subsequent function call.
 
+# %PROTOCOLS%
+
 # EXAMPLE
 
 ~~~c
@@ -100,6 +102,8 @@ int main(void)
 }
 ~~~
 
+# %AVAILABILITY%
+
 # RETURN VALUE
 
 **CURLMcode** type, general libcurl multi interface error code. See