Add a note on what happens if the "-p" option is specified

and a final directory already exists.

Reviewed by:	freebsd-audit
MFC after:	5 days

bin/mkdir/mkdir.1

@@ -73,6 +73,8 @@ are interpreted relative to an initial mode of
 Create intermediate directories as required.
 If this option is not specified, the full path prefix of each
 operand must already exist.
+On the other hand, with this option specified, no error will
+be reported if a directory given as an operand already exists.
 Intermediate directories are created with permission bits of
 .Li rwxrwxrwx (\&0777)
 as modified by the current umask, plus write and search