RFR: JDK-8268088: Clarify Method::clear_jmethod_ids() related comments in ClassLoaderData::~ClassLoaderData()

[Jiangli Zhou]

This PR clarifies Method::clear_jmethod_ids() related comments in ClassLoaderData::~ClassLoaderData(). It adds more explanations on why Method::clear_jmethod_ids only sets the jmethod_ids to NULL without releasing the memory for related JNIMethodBlocks and JNIMethodBlockNodes.

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Commit messages:
 - JDK-8268088

Changes: https://git.openjdk.java.net/jdk/pull/4383/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4383&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268088
  Stats: 11 lines in 1 file changed: 5 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4383.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4383/head:pull/4383

PR: https://git.openjdk.java.net/jdk/pull/4383