Document environment variables

[steveklabnik]

Fixes#16330

My troff is probably poor, so make sure that all looks okay.

[rust-highfive]

r? @pcwalton

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[alexcrichton]

This seems like a somewhat unstable option, so perhaps it (and RUST_REGION_GRAPH below) could be left out?

[steveklabnik]

@alexcrichton all nits should be addressed. I removed THREADs, but may want to double check that we really do real quick.

[alexcrichton]

r=me with removal of RUST_REGION_GRAPH_NODE and RUST_REGION_GRAPH

[steveklabnik]

@bors: r=alexcrichton rollup

[bors]

📌 Commit eb032af has been approved by alexcrichton

[huonw]

TASKS -> THREADS given #23525 ?

[steveklabnik]

ah yes, that got approved

[steveklabnik]

@bors: r=alexcrichton rollup

[bors]

📌 Commit b4a72a4 has been approved by alexcrichton

[sanxiyn]

According to man-pages(7), ENVIRONMENT is the traditional heading, not ENVIRONMENT VARIABLES. I think there is no need to deviate from the convention.