Improve documentation for benchmarks

[real-or-random]

A follow-up PR could add a --help which would also mention the get_iters environment variable (I have to look up in the source how it's called exactly every time).

Originally posted by @jonasnick in #991 (review)

We could also remove the indirection in cirrus.sh (https://github.com/bitcoin-core/secp256k1/blob/master/ci/cirrus.sh#L35-L37) and define the "right" constants directly in cirrus.yml. I tend to look the constants up in cirrus.yml and then see the wrong constants without the SECP256K1_ prefix.

[siv2r]

mention the get_iters environment variable

I don't understand. Do we have to mention the value of the iters variable (stores the value of get_iters func)? Or the value of SECP256K1_BENCH_ITERS?

[jonasnick]

I was just suggesting to mention the string SECP256K1_BENCH_ITERS in case someone would want to use a custom number of iterations.

[siv2r]

Ahh, got it!