Documentation of BigInteger(String) constructor does not mention leading plus

Summary

Correct the BigInteger​(String val) to state its long-standing acceptance of a leading + as valid input.

Problem

The spec changes for JDK-5017980 omitted the update for one of the two BigInteger constructors taking a string as input.

Solution

Update the spec of BigInteger​(String val) to match its long-standing behavior of accepting a leading +.

Specification

diff -r 7a3522ab48b3 src/java.base/share/classes/java/math/BigInteger.java
--- a/src/java.base/share/classes/java/math/BigInteger.java Wed Aug 05 23:02:22 2020 -0700
+++ b/src/java.base/share/classes/java/math/BigInteger.java Wed Aug 05 23:18:11 2020 -0700
@@ -659,7 +659,7 @@

     /**
      * Translates the decimal String representation of a BigInteger into a
-     * BigInteger.  The String representation consists of an optional minus
+     * BigInteger.  The String representation consists of an optional minus or plus 
      * sign followed by a sequence of one or more decimal digits.  The
      * character-to-digit mapping is provided by {@code Character.digit}.
      * The String may not contain any extraneous characters (whitespace, for