`symbol_names` is documented in a few places as only defaulting to `rule` entries

https://github.com/bazelbuild/stardoc/blob/master/docs/stardoc_rule.md#stardoc-symbol_names

https://github.com/bazelbuild/stardoc/blob/master/docs/generating_stardoc.md#single-file

Seem to say if nothing is provide for `symbol_names`, it defaults to only documenting the `rule` entries.

But https://github.com/bazelbuild/stardoc/blob/master/docs/writing_stardoc.md does say:

> Stardoc will, by default, generate documentation for all rules, macros, and functions reachable from a target `.bzl` file.

But then links over to the other doc which just talks about rules again.

https://github.com/bazelbuild/stardoc/blob/master/docs/writing_stardoc.md#rule-documentation also mentioned that attributes with a leading underscore (`_`) won't be documented, but nothing indicates if the default behavior for `symbol_names` will do something similar (skipping runes, macros, or providers that start with a underscore).

Compare the `symbol_names` sections in `docs/stardoc_rule.md` and `docs/generating_stardoc.md` with `docs/writing_stardoc.md`. Check the documented default behavior for rules, macros, functions, providers, and leading-underscore symbols. Done when the affected documentation consistently describes the behavior and links do not contradict one another.

Written by the indexing model from the issue text.