Windows permission documentation

[redlars]

We found the description of filesystem perms enumerator [1] confussing and started a discussion on boost mailinglist [2]. It appears the filesystem uses the readonly attribute to set permissions and does not use Windows ACL at all. We believe that the current docs should be updated to give a better description of the permissions functionality.

We also believe this windows warning[1] is somewhat hidden and it would be usefull if this warning was included in the permissons description [3].

[1] - https://www.boost.org/doc/libs/1_69_0/libs/filesystem/doc/reference.html#Enum-perms
[2] - https://lists.boost.org/boost-users/2018/12/89232.php
[3] - https://www.boost.org/doc/libs/1_69_0/libs/filesystem/doc/reference.html#permissions

kind regards, Lars

[Lastique]

I believe the note in the docs is adequate. If you have a suggestion on how to improve it, please create a PR.