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_EFFECTIVE_URL.md

@@ -36,6 +36,8 @@ The **urlp** pointer is NULL or points to private memory. You MUST NOT free
 - it gets freed when you call curl_easy_cleanup(3) on the corresponding
 CURL handle.
 
+# %PROTOCOLS%
+
 # EXAMPLE
 
 ~~~c
@@ -58,6 +60,8 @@ int main(void)
 }
 ~~~
 
+# %AVAILABILITY%
+
 # RETURN VALUE
 
 Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.