docs/libcurl/opts: clarify the return values

Expand a little.

- mention the type name of the return code
- avoid stating which exact return codes that might be returned, as that
  varies over time, builds and conditions
- avoid stating some always return OK
- refer to the manpage documenting all the return codes

Closes #15900

docs/libcurl/opts/CURLOPT_POST.md

@@ -98,4 +98,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if HTTP is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_setopt(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).