Clarify usage comment for set() functions (#67)

It was not immediately clear that supplying a value of false for the "exists" argument has a special meaning. It causes the "NX" argument to be sent to Redis, as opposed to no argument when supplying a value of nil. At least that was my experience when using this function for the first time - the implications of true/false/nil were not explained until I read the source code for this function.

Sources/SwiftRedis/Redis+Basic.swift

@@ -604,7 +604,7 @@ extension Redis {
     ///
     /// - Parameter key: The key.
     /// - Parameter value: The String value to set.
-    /// - Parameter exists: If true will only set the key if it already exists.
+    /// - Parameter exists: If true will only set the key if it already exists. If false will only set the key if it doesn't already exist.
     /// - Parameter expiresIn: If not nil, the expiration time, in milliseconds.
     /// - Parameter callback: The callback function after setting the value. Bool will be
     ///                      true if the key was set. NSError will be non-nil if an error occurred.
@@ -628,7 +628,7 @@ extension Redis {
     ///
     /// - Parameter key: The key.
     /// - Parameter value: The `RedisString` value to set.
-    /// - Parameter exists: If true will only set the key if it already exists.
+    /// - Parameter exists: If true will only set the key if it already exists. If false will only set the key if it doesn't already exist.
     /// - Parameter expiresIn: If not nil, the expiration time, in milliseconds.
     /// - Parameter callback: The callback function after setting the value. Bool will be
     ///                      true if the key was set. NSError will be non-nil if an error occurred.