mention how to make a curl handle use the share

docs/libcurl/curl_share_init.3

@@ -13,6 +13,10 @@ This function returns a CURLSH handle to be used as input to all the other
 share-functions, sometimes refered to as a share handle on some places in the
 documentation. This init call MUST have a corresponding call to
 \fIcurl_share_cleanup\fP when all operations using the share are complete.
+
+This \fIshare handle\fP is what you pass to curl using the \fICURLOPT_SHARE\fP
+option with \fIcurl_easy_setopt(3)\fP, to make that specific curl handle use
+the data in this share.
 .SH RETURN VALUE
 If this function returns NULL, something went wrong (out of memory, etc.)
 and therefore the share object was not created.