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/CURLOPT_NETRC.md

@@ -118,6 +118,8 @@ done with "macdef" that it finds.
 
 CURL_NETRC_IGNORED
 
+# %PROTOCOLS%
+
 # EXAMPLE
 
 ~~~c
@@ -133,6 +135,8 @@ int main(void)
 }
 ~~~
 
+# %AVAILABILITY%
+
 # RETURN VALUE
 
 Returns CURLE_OK