rustdoc: add missing {os,target,target_env} values for cfg pretty printer

[scrabsha]

(draft because i want to see what CI says of this before bothering people. i don't really know who to ask review to, feel free to assign yourself)

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

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======== tests/rustdoc-gui/item-info-overflow.goml ========

[ERROR] line 9: The following errors happened: [`Available on Android or Linux or Emscripten or DragonFlyBSD or FreeBSD or NetBSD or OpenBSD only.` doesn't start with `Available on Android or Linux or Emscripten or DragonFly BSD` (for STARTS_WITH check)]: for command `assert-text: (
    ".item-info",
    "Available on Android or Linux or Emscripten or DragonFly BSD",
    STARTS_WITH,
)`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/lib2/struct.LongItemInfo.html>
[ERROR] line 27: The following errors happened: [`Available on Android or Linux or Emscripten or DragonFlyBSD or FreeBSD or NetBSD or OpenBSD only.` doesn't start with `Available on Android or Linux or Emscripten or DragonFly BSD` (for STARTS_WITH check)]: for command `assert-text: (
    "#impl-SimpleTrait-for-LongItemInfo2 .item-info",
    "Available on Android or Linux or Emscripten or DragonFly BSD",
    STARTS_WITH,
)`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/lib2/struct.LongItemInfo2.html>