RFR: JDK-8268088: Clarify Method::clear_jmethod_ids() related comments in ClassLoaderData::~ClassLoaderData() [v2]

[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.

Jiangli Zhou has updated the pull request incrementally with two additional commits since the last revision:

 - Remove whitespace.
 - Updated based on Evgeny's comments.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4383/files
  - new: https://git.openjdk.java.net/jdk/pull/4383/files/2ae0b3f0..17ddc32f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4383&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4383&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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