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_remove_handle.md

@@ -47,6 +47,8 @@ libcurl keeps the connection alive in the connection pool associated with the
 multi handle, ready to get reused for a future transfer using this multi
 handle.
 
+# %PROTOCOLS%
+
 # EXAMPLE
 
 ~~~c
@@ -67,6 +69,8 @@ int main(void)
 }
 ~~~
 
+# %AVAILABILITY%
+
 # RETURN VALUE
 
 CURLMcode type, general libcurl multi interface error code.