curl_version_info.3: returns a pointer to a static struct

And clarify that age 3 means 7.16.1 or later.

docs/libcurl/curl_version_info.3

@@ -29,7 +29,7 @@ curl_version_info - returns run-time libcurl version info
 .BI "curl_version_info_data *curl_version_info( CURLversion "type ");"
 .ad
 .SH DESCRIPTION
-Returns a pointer to a filled in struct with information about various
+Returns a pointer to a filled in static struct with information about various
 features in the running version of libcurl. \fItype\fP should be set to the
 version of this functionality by the time you write your program. This way,
 libcurl will always return a proper struct that your program understands,
@@ -66,7 +66,8 @@ typedef struct {
   /* when 'age' is 2 or higher, the member below also exists: */
   const char *libidn;       /* human readable string */
 
-  /* when 'age' is 3 or higher, the members below also exist: */
+  /* when 'age' is 3 or higher (7.16.1 or later), the members below also
+     exist  */
   int iconv_ver_num;       /* '_libiconv_version' if iconv support enabled */
 
   const char *libssh_version; /* human readable string */