Auto merge of #30807 - kolloch:patch-1, r=steveklabnik

When I read the book, the following sentence of the reference documentation helped me greatly to understand slices:

"Slices are a view into a block of memory represented as a pointer and a length."

In this commit, I tried to integrate the gist of that into the slice section inside of "primitive-types.md". I am not a native speaker, so feel very free to improve the wording.

r? @steveklabnik

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