Allow to provide a custom map of rendered strings for `doc(cfg(...))`

[ojeda]

Currently, rustdoc has some hardcoded logic to render doc(cfg(...)) in:

rust/src/librustdoc/clean/cfg.rs

Line 464 in 4f0c568

Cfg::Cfg(name, value) => {

rustdoc could also take a map of config options to "rendered strings" (passed as e.g. a file via the command line), so that one may have:

This is supported on Networking only.

instead of:

This is supported on CONFIG_NET only.

Ideally, there could be also be one for the "short" name (that goes in the index) and the "long" name (that goes inside the item); and perhaps even allow for Markdown and to provide a custom formatting string (so that the message can be customized), e.g. it could be used to have something like:

Supported when My Driver (CONFIG_MY_DRIVER) is enabled.
Supported when My Driver (CONFIG_MY_DRIVER) is enabled as a module (M).
Supported when My Driver (CONFIG_MY_DRIVER) is enabled as a built-in (Y).

[lolbinarycat]

this could also be used to help mitigate #131735