RFR: 8196609: Improve javadoc for java.time.Instant.getEpochSecond

[Martin Buchholz]

The nanos *are* hard to describe non-confusingly - resisting the urge to
fix more globally.

--- a/src/java.base/share/classes/java/time/Instant.java
+++ b/src/java.base/share/classes/java/time/Instant.java
@@ -610,7 +610,7 @@
      * <p>
      * The epoch second count is a simple incrementing count of seconds
where
      * second 0 is 1970-01-01T00:00:00Z.
-     * The nanosecond part of the day is returned by {@link #getNano}.
+     * The nanosecond part is returned by {@link #getNano}.
      *
      * @return the seconds from the epoch of 1970-01-01T00:00:00Z
      */