chore(docs): Add ELECTRIC_FEATURES environment variable to CLI

[msfstef]

To be merged once #1304 lands in main.

[samwillis]

@msfstef @magnetised is there a docs PR for the new ELECTRIC_FEATURES env var? These cli docs for the env var should link to the docs for the server env var and how it can be used.

[msfstef]

@samwillis you're right I don't see this in the docs anywhere but it is documented in code:

electric/components/electric/lib/electric/features.ex

Lines 11 to 18 in 25523d9

	2. Via an env variable `ELECTRIC_FEATURES`. This uses a PATH-like syntax,
	where flag settings are separated by colons, e.g. to add or update the
	existing environment configuration with `another_feature` off and
	`some_feature` on, you would do:
	export ELECTRIC_FEATURES="another_feature=false:some_feature=true:${ELECTRIC_FEATURES:-}"
	i.e. values at the start of the env variable have precedence.

[samwillis]

@msfstef Maybe this PR can add both parts of the docs?

[msfstef]

@samwillis I think it should but I'm hoping for someone more familiar with this flag to confirm it haha @magnetised @alco ?

I've added it tentatively here bbb8ea3