GH-50087: [Docs][C++] [Docs][C++] Fix sentence structure in memory.rst regarding MemoryManager

[OmBiradar]

In file docs/source/cpp/memory.rst:

• Removes a duplicate line
• Minor grammar fix

Rationale for this change

I believe the statement

The associated class :class:arrow::MemoryManager specifies how to allocate on a given device.

was repeated twice and there was some grammatical mistake in the same paragraph

• GitHub Issue: [Docs][C++] Fix sentence structure in memory.rst regarding MemoryManager #50087

[github-actions]

⚠️ GitHub issue #50087 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

This PR cleans up the C++ memory documentation by removing a duplicated sentence fragment in the arrow::Device / arrow::MemoryManager explanation, improving readability without changing meaning.

Changes:

• Removed a duplicated/redundant description of arrow::MemoryManager allocation behavior.
• Left the remaining paragraph with a single, clear example referencing arrow::MemoryPool on the CPU.

[OmBiradar]

cc: @amoeba

[amoeba]

Hey @OmBiradar, thanks for the PR. The patch looks good.

Before we approve and merge, could you make some changes to the issue title and PR title to help maintainers and future contributors out?

• I already changed the PR title a bit:
  • I changed [DOCS] to [Docs] just to match the style used in other PRs. Using existing issues and PRs as a guide for new ones is often a good idea.
  • I removed the [MINOR] tag since we don't use that tag when the PR has an associated issue. Filing an issue like you did is always a good step if you aren't sure but, now that you have some more experience, changes like this are fine as minor PRs and filing an issue first isn't required.
  • I added [C++] since it helps to clarify this Docs change is to the C++ docs.
• Can you improve the wording of the PR title? "Fix the docs of arrow::MemoryManager" isn't specific enough. PR titles will show up in the git blame so something specific would be better. For example, "Remove misplaced sentence fragment in memory.rst"
• Can you improve the issue title? Something more specific (like above) would be good.

[OmBiradar]

Hey @amoeba I have changed the titles of both the PR and the Issue. Let me know if its ok.

I will keep in mind the points you mentioned in all my future PR's and issues raised.

Thanks for the help!