mention RFC 2396 for URL syntax spec

docs/libcurl/curl_easy_setopt.3

@@ -295,7 +295,10 @@ given protocol of the set URL is not supported, libcurl will return on error
 \fIcurl_multi_perform(3)\fP. Use \fIcurl_version_info(3)\fP for detailed info
 on which protocols that are supported.
 
-\fICURLOPT_URL\fP is the only option that must be set before
+The string given to CURLOPT_URL must be url-encoded and following the RFC 2396
+(http://curl.haxx.se/rfc/rfc2396.txt).
+
+\fICURLOPT_URL\fP is the only option that \fBmust\fP be set before
 \fIcurl_easy_perform(3)\fP is called.
 .IP CURLOPT_PROXY
 Set HTTP proxy to use. The parameter should be a char * to a zero terminated