docs: Updates the documentation for the wepKeys field.

Updated documentation indicates that four WEP key values must be
provided. This replaces the earlier reference to "up to four." Providing
fewer than four values causes a java.lang.IllegalArgumentException.

Documentation also references the option to provide empty strings for up
to three of the values.

Test: make ds-docs

Bug: 36949880

Change-Id: I1da9f0ecfb390623f4d53ca508a554303861ed98
Exempt-From-Owner-Approval: Docs-only change

wifi/java/android/net/wifi/WifiConfiguration.java

@@ -328,9 +328,10 @@ public class WifiConfiguration implements Parcelable {
     public String preSharedKey;
 
     /**
-     * Up to four WEP keys. Either an ASCII string enclosed in double
-     * quotation marks (e.g., {@code "abcdef"}) or a string
-     * of hex digits (e.g., {@code 0102030405}).
+     * Four WEP keys. For each of the four values, provide either an ASCII
+     * string enclosed in double quotation marks (e.g., {@code "abcdef"}),
+     * a string of hex digits (e.g., {@code 0102030405}), or an empty string
+     * (e.g., {@code ""}).
      * <p/>
      * When the value of one of these keys is read, the actual key is
      * not returned, just a "*" if the key has a value, or the null