RFR: 8261670: Add javadoc for the XML processing limits

[Joe Wang]

Add the documentation for XML processing limits to module summary. The limits were previously documented in Java tutorial and guide.

-------------

Commit messages:
 - 8261670: Add javadoc for the XML processing limits

Changes: https://git.openjdk.java.net/jdk/pull/2732/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2732&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261670
  Stats: 87 lines in 1 file changed: 81 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2732.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2732/head:pull/2732

PR: https://git.openjdk.java.net/jdk/pull/2732