docs: clarify let: directives

[jamesscottbrown]

This tries to clarify the explanation of let: directives through 3 changes:

• use a name other than prop for the specific prop in the example. Using the generic name prop might make a reader wonder if this is a keyword or magic name choice that needs to be used
• add some comments to the example
• re-phrases the explanation to avoid the reader having to think about which direction is backwards

I found the statement "Slots can be rendered zero or more times and can pass values back to the parent" initially unclear because there are two dfferent child-parent relationships to consider (a component definition with a child <slot>, and a use of the component with a child slot template).

I think these changes are helpful because this syntax is inherently somewhat confusing: as noted in https://github.com/sveltejs/svelte/pull/4125, let:foo={bar} defines bar to be a new variable set to the value of foo, which is counter-intuitive.

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[PuruVJ]

Hi! Could you resolve merge conflicts? The file structure has changed, so you may wanna move these changes to documentation/docs/02-template-syntax/07-special-elements.md 😁

[benmccann]

Hi @jamesscottbrown. Are you still interested in pursuing this PR? If so, the merge conflicts will need to be addressed