docs: document environment variables

[Stebalien]

fixes #5828

[michaelavila]

LGTM.

Re environment vars: I did notice while looking around that we have a reference to the environment variable "IPFS_LOW_MEM" (./go-ipfs-flags/flags.go:11: if os.Getenv("IPFS_LOW_MEM") != "" {), but I don't actually see it being used anywhere. I mention it just in case. This list otherwise looks like the relevant stuff I'm finding.

Re mux: I can't comment on the stability, but I can take your word for it. For my own benefit, can you quickly elaborate on this experiment?

[Stebalien]

Re environment vars: I did notice while looking around that we have a reference to the environment variable "IPFS_LOW_MEM" (./go-ipfs-flags/flags.go:11: if os.Getenv("IPFS_LOW_MEM") != "" {), but I don't actually see it being used anywhere. I mention it just in case. This list otherwise looks like the relevant stuff I'm finding.

It looks like it is used by bitswap.

Re mux: I can't comment on the stability, but I can take your word for it. For my own benefit, can you quickly elaborate on this experiment?

We have two stream multiplexers: yamux and mplex. We use yamux by default but neither js-libp2p nor rust-libp2p support yamux so we fallback on mplex.

[Stebalien]

IPFS_LOW_MEM

Removed in ipfs/go-bitswap#115.

1. This currently increases memory usage.
2. We should just make this a configuration option.