Clarify actual and desired size in AllocationManager

[asfimport]

As a follow up to the review of ARROW-7329, we should clarify the different sizes (desired vs actual size) in AllocationManager: #5973 (comment)

Reporter: David Li / @lidavidm
Assignee: Rong Rong

Related issues:

• [Java] AllocationManager: Allow managing different types of memory other than those are allocated using Netty (relates to)

PRs and other links:

• GitHub Pull Request #6204

_{Note: This issue was originally created as ARROW-7538. Please see the migration documentation for further details.}

[asfimport]

Igor Yastrebov:
@lidavidm [~emkornfield@gmail.com] is this resolved?

[asfimport]

David Li / @lidavidm:
Resolved by pull request #6204.