RFR: 8325748: Serial: Move Generation::promote to TenuredGeneration

[Albert Mingkun Yang]

Trivial moving a method, and minor doc revision.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17830/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17830&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325748
  Stats: 78 lines in 4 files changed: 38 ins; 40 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17830.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17830/head:pull/17830

PR: https://git.openjdk.org/jdk/pull/17830