Documentation for `extend` doesn't accurately describe `lint.ignore` and `lint.select` behavior

[chirizxc]

The current documentation for the extend configuration option states:

"To resolve the current pyproject.toml file, Ruff will first resolve this base configuration file, then merge in any properties defined in the current configuration file."

This description is misleading for lint.ignore and lint.select options, as their behavior depends on whether select is specified in the child configuration.

Actual Behavior:

1. Without select in child config (code)

When select is not specified in the child config, rule_selections from parent and child configs are accumulated into a vector, not replaced.

# Parent config  
[lint]
select = ["ALL"]
ignore = ["E501"]

# Child config
extend = "../ruff.toml"
[lint]
ignore = ["F401"]

# Result: Both E501 AND F401 are ignored (accumulated)

2. With select in child config (code)

When select is specified in the child config, it triggers a complete reset of select_set, and parent's ignore rules are not applied.

# Parent config
[lint]
select = ["ALL"]
ignore = ["E501"]

# Child config
extend = "../ruff.toml"
[lint]
select = ["EM", "TC"]
ignore = ["EM102", "TC001"]

# Result: Only EM102 and TC001 is ignored (parent's E501 is NOT ignored)

[MichaReiser]

Thanks for opening this issue. We're aware that the select and extend- semantic is somewhat confusing and we're thinking on replacing it with a simpler but more consistent behavior. But that doesn't mean that we can't improve our documentation in the meantime.

I think it makes sense to add a link to the Rule selection page and copy the following paragraph (and adjust it)

In those scenarios, Ruff uses the "highest-priority" select as the basis for the rule set, and then applies extend-select and ignore adjustments. CLI options are given higher priority than pyproject.toml options, and the current pyproject.toml file is given higher priority than any inherited pyproject.toml files.