implementing-dispose.md: Better illustration of SafeHandles

[antonfirsov]

Implement the plan described in #44201 (comment) and apply some further changes:

• Use MemoryStream to demonstrate managed only dispose pattern
• For the unmanaged dispose pattern, actually allocate a handle
• Define a new example for working with SafeHandle in the Safe handles paragraph
• Reshuffle the order of paragraphs so it aligns with the new content better
• An adjustment on Clarify ownership of IDisposable objects #34144, changing the text in "Cascade dispose calls" that talks about ownership. (There is no such rule that only the members created by the class should be disposed by the class, sometimes ownership can be transferred to another IDisposable.)

Fixes #44201

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docs/standard/garbage-collection/implementing-dispose.md	Implement a Dispose method

[BillWagner]

Thanks again @antonfirsov

This looks great, and I'll now.

[gewarren]

I found a few more nits.