Update env var advice

[hadley]

i.e. don't store anyway; grab live when creating request. If provide user facing function, note weakrefs

[jonthegeek]

I remember reading about weakrefs in the DSLC.io {rlang} "book" club (https://youtu.be/q78fjUUAgA4), but I didn't grok when to use/think about them. I'm excited to read this advice!