WifiConfiguration: Update psk documentation

Adding some documentation to explain the expected contents of this
field.

Bug: 35967623
Test: make update-api
Change-Id: Ia28e2c79834c4d7b0de7350c18d5f32633e6309d
diff --git a/wifi/java/android/net/wifi/WifiConfiguration.java b/wifi/java/android/net/wifi/WifiConfiguration.java
index a1099f8..6e659e6 100644
--- a/wifi/java/android/net/wifi/WifiConfiguration.java
+++ b/wifi/java/android/net/wifi/WifiConfiguration.java
@@ -281,7 +281,9 @@
     public int apChannel = 0;
 
     /**
-     * Pre-shared key for use with WPA-PSK.
+     * Pre-shared key for use with WPA-PSK. Either an ASCII string enclosed in
+     * double quotation marks (e.g., {@code "abcdefghij"} for PSK passphrase or
+     * a string of 64 hex digits for raw PSK.
      * <p/>
      * When the value of this key is read, the actual key is
      * not returned, just a "*" if the key has a value, or the null