(pedantic) The `box_collection` docs are not quite correct.

[Lokathor]

Description

Currently the docs for box_collection state:

## Why is this bad?
Collections already keeps their contents in a separate area on the heap. So if you Box them, you just add another level of indirection without any benefit whatsoever.

However, there is one clear benefit: Box is one non-null pointer big instead of being 2-3 (or more!) pointers big. Particularly for error data, the extra layer of boxing to keep the error variant small outweighs the mild speed penalty.

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[GeeWee]

Stumbled upon this as we faced the exact same thing with regards to errors, where we'd get the large_enum_variant error as our errors are bigger - we'd then box collections inside the error to get under the size threshold, and then get this new clippy error instead.