RFR: 8323159: Consider adding some text re. memory zeroing in Arena::allocate [v2]

[Per Minborg]

> This PR proposes to add a clarification that an `Arena` always returns zeroed-out segments for `Arena::allocate` methods.
> 
> Note that other overloaded methods refer to the abstract `Arena::allocate` method via implementation notes.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Move and change text on zeroing out

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17308/files
  - new: https://git.openjdk.org/jdk/pull/17308/files/4c777398..04074d71

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17308&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17308&range=00-01

  Stats: 9 lines in 1 file changed: 5 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17308.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17308/head:pull/17308

PR: https://git.openjdk.org/jdk/pull/17308