RFR: 8289434: x86_64: Improve comment on gen_continuation_enter()

[Richard Reingruber]

Change code comments for `gen_continuation_enter()` explaining that the generated code will call `Continuation.enter(Continuation c, boolean isContinue)` if the continuation give as first parameter is run for the first time.

Also mention the special case for resolving this call.

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

Commit messages:
 - 8289434: Improve comment on gen_continuation_enter()

Changes: https://git.openjdk.org/jdk/pull/9320/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9320&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289434
  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9320.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9320/head:pull/9320

PR: https://git.openjdk.org/jdk/pull/9320