Update docs about deleting extra fields.

man/zip_file_extra_field_delete.mdoc

diff --git a/man/zip_file_extra_field_delete.mdoc b/man/zip_file_extra_field_delete.mdoc
index 61cd709..2581355 100644
--- a/man/zip_file_extra_field_delete.mdoc
+++ b/man/zip_file_extra_field_delete.mdoc
@@ -29,7 +29,7 @@
 .\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
 .\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd February 20, 2013
+.Dd July 31, 2013
 .Dt ZIP_FILE_EXTRA_FIELD_DELETE 3
 .Os
 .Sh NAME
@@ -56,7 +56,7 @@
 in the zip archive.
 .Pp
 If
-.Ar index
+.Ar extra_field_index
 is
 .Dv ZIP_EXTRA_FIELD_ALL ,
 then all extra fields will be deleted.
@@ -84,7 +84,21 @@
 The other arguments are the same as for
 .Fn zip_file_extra_field_delete
 .Dv ( ZIP_EXTRA_FIELD_ALL
-will delete all extra fields the specified ID).
+will delete all extra fields of the specified ID).
+.Pp
+Please note that due to the library design, the index of an extra
+field may be different between central directory and local file
+headers.
+For this reason, it is not allowed to specify both
+.Dv ZIP_FL_CENTRAL
+and
+.Dv ZIP_FL_LOCAL
+in
+.Ar flags ,
+except when deleting all extra fields (i.e.,
+.Ar extra_field_index
+being
+.Dv ZIP_EXTRA_FIELD_ALL ) .
 .Sh RETURN VALUES
 Upon successful completion 0 is returned.
 Otherwise, \-1 is returned and the error code in