Incorporate ideas from StackOverflow cell/reference answer into std::cell docs

[frewsxcv]

https://www.reddit.com/r/rust/comments/6tma1a/a_great_so_answer_about_stdcell/

https://stackoverflow.com/questions/45674479/need-holistic-explanation-about-rusts-cell-and-reference-counted-types/45674912#45674912

EDIT: also this

[est31]

Without consent from @matthieu-m there might be license issues, as rust docs are under the Apache2/MIT dual license and stack overflow posts are CC-BY-SA.

[matthieu-m]

@est31: I am perfectly happy to give consent, and for the content to be reused in whole or in parts, altered or not, and without citation (as it would I think disturb the flow of reading).

[seanprashad]

Hi there! I would love to help work on this bug if it's still available!

[frewsxcv]

@seanprashad yep, it's still available! if you have any questions or need any help, don't hesitate to ask!

[frewsxcv]

@seanprashad did you make any progress on this or is it open for someone else to start work on it?

[seanprashad]

@frewsxcv Unfortunately I wasn't able to - please open it for other individuals to take a shot

[pvdrz]

If its ok I would like to do this

[frewsxcv]

Sounds great @christianpoveda! If you want any help or don't know where to start or have any questions, don't hesitate to ask!