(Re)document the `RFM_PIPELINE_TIMEOUT` environment variable

[vkarak]

Although the the general.pipeline_timeout config option is documented, the associated environment variable is not. And we should probably give more context on how to set this variable as this has arisen in three different occasions in Slack discussions:

• https://reframetalk.slack.com/archives/CF8ENLUDB/p1670183911574269
• https://reframetalk.slack.com/archives/CF8ENLUDB/p1669715576916539
• https://reframetalk.slack.com/archives/CF8ENLUDB/p1673294294007029?thread_ts=1673218130.537729&cid=CF8ENLUDB