[libcxx] Document windows specifics regarding file_type and perms. NFC.

[mstorsjo]

Summary

This was requested in the review of D98138.

[• Quuxplusone]

LGTM, fwiw.

[curdeius]

LGTM.

libcxx/include/filesystem

 typedef chrono::time_point<_FilesystemClock> file_time_type;
 
 struct _LIBCPP_TYPE_VIS space_info {
   uintmax_t capacity;
   uintmax_t free;
   uintmax_t available;
 };
 
+// On Windows, the library never identifies files as  block, character, fifo
+// or socket.
 enum class _LIBCPP_ENUM_VIS file_type : signed char {
   none = 0,
   not_found = -1,
   regular = 1,
   directory = 2,
   symlink = 3,
   block = 4,
   character = 5,
   fifo = 6,
   socket = 7,
   unknown = 8
 };
 
+// On Windows, these permission bits map to one single readonly flag per
+// file, and the executable bit is always returned as set. When setting
+// permissions, as long as the write bit is set for either owner, group or
+// others, the readonly flag is cleared.
 enum class _LIBCPP_ENUM_VIS perms : unsigned {
   none = 0,
 
   owner_read = 0400,
   owner_write = 0200,
   owner_exec = 0100,
   owner_all = 0700,