Documentation improvements

[tatomyr]

This is a placeholder for relatively small and low-priority CLI documentation issues. Will keep it open and update as I find more.

• improve split documentation so it clarifies the intended usage (split command does not support plugins #1822)
• explain how to use openapi-core features in JS - https://github.com/Redocly/redocly-cli/blob/main/packages/core/README.md
• add explanation about overriding severity of configurable rules
• improve eject and other realm-related commands docs (see Ejected components and templates fail with an error about useLocation #1847 (comment))
• clarify the filter-in decorator behaviour (filter-in on tags includes operations that don't have a tag  #1741)
• clarify the merging stragety of root and apis-level config (Merge-resolve per-API with root configuration not fully supported #1871 (comment))
• clarify that join doesn't run decorators (Can't use config with the join command #1889 (comment)) since fix: do not execute decorators and preprocessors in join command #1046 and feat: remove --lint, --preprocess, and --decorate options from the join command #1507
• TBD

cc @JLekawa