[ doc ] Improve docs for let and :=

[CodingCellist]

Description

Prompted by discussion on the Discord: there was confusion as to when computations would be re-evaluated, whether type annotations made a difference (and when), the difference between let with = and with :=.

The string for :doc (:=) was not very helpful; only a single line mentioning what the symbol could be used for. This has now been expanded, and the expanded version lifted out to its own string so as to be (re)used in both := and let's documentation.

Should this change go in the CHANGELOG?

• [x] If this is a fix, user-facing change, a compiler change, or a new paper implementation, I have updated CHANGELOG.md (and potentially also CONTRIBUTORS.md).

[emdash]

Thanks for doing this. I am not officially a contributor here, but it gets my vote.

[CodingCellist]

@emdash @buzden, thank you both for looking this over! I think I've incorporated the feedback, let me know what yous think (after the holidays if you're away; please don't take time away from a break to look at this) : )

[mattpolzin]

~The changelog entry on this PR will need to be updated now that v0.7.0 was released.~ [Done]