curl_easy_setopt.3: mention how to unset CURLOPT_INFILESIZE*

docs/libcurl/curl_easy_setopt.3

@@ -2134,6 +2134,8 @@ as a long. See also \fICURLOPT_INFILESIZE_LARGE\fP.
 For uploading using SCP, this option or \fICURLOPT_INFILESIZE_LARGE\fP is
 mandatory.
 
+To "unset" this value again, set it to -1.
+
 When sending emails using SMTP, this command can be used to specify the
 optional SIZE parameter for the MAIL FROM command. (Added in 7.23.0)
 
@@ -2146,6 +2148,8 @@ as a curl_off_t. (Added in 7.11.0)
 
 For uploading using SCP, this option or \fICURLOPT_INFILESIZE\fP is mandatory.
 
+To "unset" this value again, set it to -1.
+
 When sending emails using SMTP, this command can be used to specify the
 optional SIZE parameter for the MAIL FROM command. (Added in 7.23.0)