Update Perm docs

Author: hdgarrood

docs/Node/FS/Perms.md

@@ -17,7 +17,9 @@ allows you to combine permissions:
    permissions that two `Perm` values have in common. For example:
    `(read + write) * (write + execute) == write`.
 
-
+You can think also of a `Perm` value as a subset of the set
+`{ read, write, execute }`; then, `(+)` and `(*)` represent set union and
+intersection respectively.
 
 ##### Instances
 ``` purescript

src/Node/FS/Perms.purs

@@ -31,7 +31,9 @@ import Data.Int (fromNumber, toNumber)
 -- |    permissions that two `Perm` values have in common. For example:
 -- |    `(read + write) * (write + execute) == write`.
 -- |
--- | 
+-- | You can think also of a `Perm` value as a subset of the set
+-- | `{ read, write, execute }`; then, `(+)` and `(*)` represent set union and
+-- | intersection respectively.
 newtype Perm = Perm { r :: Boolean, w :: Boolean, x :: Boolean }
 
 instance eqPerm :: Eq Perm where