[[ bugfix-13642 ]] Improve the documentation for the umask property.

When setting (or retrieving) the umask, it is convenient to use octal
representation, and this is what the documentation currently
recommends.

Unfortunately, LiveCode won't convert base-8 literal integers by
default, so at the moment following the instructions in the
documentation and writing "set the umask to 077" gives a real umask of
0115 (octal).

Update the documentation to clarify this a bit.