RFR: 8293781: RISC-V: Clarify types of calls

[Yanhong Zhu]

RISCV64 OpenJDK uses different types of calls in generated calls. It should be clarified what they are. And the PR also cleans up some out-of-date comments.

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

Commit messages:
 - Clarify types of calls

Changes: https://git.openjdk.org/jdk/pull/10311/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10311&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293781
  Stats: 79 lines in 3 files changed: 70 ins; 8 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10311.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10311/head:pull/10311

PR: https://git.openjdk.org/jdk/pull/10311