[SPARK-33115][BUILD][DOCS] Fix javadoc errors in kvstore and `unsaf…

…e` modules

### What changes were proposed in this pull request?

Fix Javadoc generation errors in `kvstore` and `unsafe` modules according to error message hints.

### Why are the changes needed?

Fixes `doc` task failures which prevented other tasks successful executions (eg `publishLocal` task depends on `doc` task).

### Does this PR introduce _any_ user-facing change?

No.
Meaning of text in Javadoc is stayed the same.

### How was this patch tested?

Run `build/sbt kvstore/Compile/doc`, `build/sbt unsafe/Compile/doc` and `build/sbt doc` without errors.

Closes #30007 from gemelen/feature/doc-task-fix.

Authored-by: Denis Pyshev <git@gemelen.net>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
(cherry picked from commit 1b0875b)
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

common/kvstore/src/main/java/org/apache/spark/util/kvstore/InMemoryStore.java

@@ -164,8 +164,9 @@ public void clear() {
   }
 
   /**
-   * An alias class for the type "ConcurrentHashMap<Comparable<Object>, Boolean>", which is used
-   * as a concurrent hashset for storing natural keys and the boolean value doesn't matter.
+   * An alias class for the type "{@literal ConcurrentHashMap<Comparable<Object>, Boolean>}",
+   * which is used as a concurrent hashset for storing natural keys
+   * and the boolean value doesn't matter.
    */
   private static class NaturalKeys extends ConcurrentHashMap<Comparable<Object>, Boolean> {}
 

common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -563,7 +563,7 @@ public UTF8String trim() {
   }
 
   /**
-   * Trims whitespaces (<= ASCII 32) from both ends of this string.
+   * Trims whitespaces ({@literal <=} ASCII 32) from both ends of this string.
    *
    * Note that, this method is the same as java's {@link String#trim}, and different from
    * {@link UTF8String#trim()} which remove only spaces(= ASCII 32) from both ends.