RFR: [15,docs] JDK-8247959,doclint errors in NIO code

[Jonathan Gibbons]

Please review some fixes to address issues found by doclint.

In a couple of places, imports were required in order to resolve symbols.
In another case, the name also had a typo in it.
In the last case, the incorrect syntax was used for a type parameter in 
@param.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8247959

Patch inline:

diff -r 086c7f077fc6 
src/jdk.nio.mapmode/share/classes/jdk/nio/mapmode/ExtendedMapMode.java
--- 
a/src/jdk.nio.mapmode/share/classes/jdk/nio/mapmode/ExtendedMapMode.java 
Fri Jun 19 15:22:19 2020 -0400
+++ 
b/src/jdk.nio.mapmode/share/classes/jdk/nio/mapmode/ExtendedMapMode.java 
Fri Jun 19 20:24:02 2020 -0700
@@ -25,6 +25,8 @@

  package jdk.nio.mapmode;

+import java.nio.MappedByteBuffer;
+import java.nio.channels.FileChannel;
  import java.nio.channels.FileChannel.MapMode;

  /**
@@ -52,7 +54,7 @@

      /**
       * File mapping mode for a read-write mapping of a file backed by
- * non-volatile RAM. {@linkplain MappedByteBufefr#force force}
+ * non-volatile RAM. {@linkplain MappedByteBuffer#force force}
       * operations on a buffer created with this mode will be performed
       * using cache line writeback rather than proceeding via a file
       * device flush.
diff -r 086c7f077fc6 
src/jdk.sctp/share/classes/com/sun/nio/sctp/NotificationHandler.java
--- 
a/src/jdk.sctp/share/classes/com/sun/nio/sctp/NotificationHandler.java 
Fri Jun 19 15:22:19 2020 -0400
+++ 
b/src/jdk.sctp/share/classes/com/sun/nio/sctp/NotificationHandler.java 
Fri Jun 19 20:24:02 2020 -0700
@@ -45,7 +45,7 @@
   * this handler interface as the type for parameters, return type, 
etc. rather
   * than the abstract class.
   *
- * @param T The type of the object attached to the receive operation
+ * @param <T> The type of the object attached to the receive operation
   *
   * @since 1.7
   */