Document surprising and dangerous fs::Permissions behaviour on Unix

This documents the very surprising behaviour that `set_readonly(false)` will make a file *world writable* on Unix. I would go so far as to say that this function should be deprecated on Unix, or maybe even entirely. But documenting the bad behaviour is a good first step.

library/std/src/fs.rs

@@ -1365,6 +1365,34 @@ impl FileTimes {
 impl Permissions {
     /// Returns `true` if these permissions describe a readonly (unwritable) file.
     ///
+    /// # Note
+    ///
+    /// This function does not take Access Control Lists (ACLs) or Unix group
+    /// membership into account.
+    ///
+    /// # Windows
+    ///
+    /// On Windows this returns [`FILE_ATTRIBUTE_READONLY`](https://docs.microsoft.com/en-us/windows/win32/fileio/file-attribute-constants).
+    /// If `FILE_ATTRIBUTE_READONLY` is set then writes to the file will fail
+    /// but the user may still have permission to change this flag. If
+    /// `FILE_ATTRIBUTE_READONLY` is *not* set then writes may still fail due
+    /// to lack of write permission.
+    /// The behavior of this attribute for directories depends on the Windows
+    /// version.
+    ///
+    /// # Unix (including macOS)
+    ///
+    /// On Unix-based platforms this checks if *any* of the owner, group or others
+    /// write permission bits are set. It does not check if the current
+    /// user is in the file's assigned group. It also does not check ACLs.
+    /// Therefore even if this returns true you may not be able to write to the
+    /// file, and vice versa. The [`PermissionsExt`] trait gives direct access
+    /// to the permission bits but also does not read ACLs. If you need to
+    /// accurately know whether or not a file is writable use the `access()`
+    /// function from libc.
+    ///
+    /// [`PermissionsExt`]: crate::os::unix::fs::PermissionsExt
+    ///
     /// # Examples
     ///
     /// ```no_run
@@ -1390,8 +1418,40 @@ impl Permissions {
     /// using the resulting `Permission` will update file permissions to allow
     /// writing.
     ///
-    /// This operation does **not** modify the filesystem. To modify the
-    /// filesystem use the [`set_permissions`] function.
+    /// This operation does **not** modify the files attributes. This only
+    /// changes the in-memory value of these attributes for this `Permissions`
+    /// instance. To modify the files attributes use the [`set_permissions`]
+    /// function which commits these attribute changes to the file.
+    ///
+    /// # Note
+    ///
+    /// `set_readonly(false)` makes the file *world-writable* on Unix.
+    /// You can use the [`PermissionsExt`] trait on Unix to avoid this issue.
+    ///
+    /// It also does not take Access Control Lists (ACLs) or Unix group
+    /// membership into account.
+    ///
+    /// # Windows
+    ///
+    /// On Windows this sets or clears [`FILE_ATTRIBUTE_READONLY`](https://docs.microsoft.com/en-us/windows/win32/fileio/file-attribute-constants).
+    /// If `FILE_ATTRIBUTE_READONLY` is set then writes to the file will fail
+    /// but the user may still have permission to change this flag. If
+    /// `FILE_ATTRIBUTE_READONLY` is *not* set then the write may still fail if
+    /// the user does not have permission to write to the file.
+    ///
+    /// In Windows 7 and earlier this attribute prevents deleting empty
+    /// directories. It does not prevent modifying the directory contents.
+    /// On later versions of Windows this attribute is ignored for directories.
+    ///
+    /// # Unix (including macOS)
+    ///
+    /// On Unix-based platforms this sets or clears the write access bit for
+    /// the owner, group *and* others, equivalent to `chmod a+w <file>`
+    /// or `chmod a-w <file>` respectively. The latter will grant write access
+    /// to all users! You can use the [`PermissionsExt`] trait on Unix
+    /// to avoid this issue.
+    ///
+    /// [`PermissionsExt`]: crate::os::unix::fs::PermissionsExt
     ///
     /// # Examples
     ///
@@ -1405,7 +1465,8 @@ impl Permissions {
     ///
     ///     permissions.set_readonly(true);
     ///
-    ///     // filesystem doesn't change
+    ///     // filesystem doesn't change, only the in memory state of the
+    ///     // readonly permission
     ///     assert_eq!(false, metadata.permissions().readonly());
     ///
     ///     // just this particular `permissions`.