kind filters are insufficently documented

[lolbinarycat]

some of these, like tymethod, i don't even know what they do.

there's a list in the formal search syntax, but there's a lot of missing descriptions.

the "search tricks" inline help also seems to have an "exhaustive" list, but is actually missing a ton of kinds.

https://doc.rust-lang.org/nightly/rustdoc/read-documentation/search.html

[fmease]

Uhh, tymethod is a relic and ideally shouldn't be exposed to the user (until now I didn't realize that it is). It denotes required associated functions inside trait declarations (i.e., ones that don't have default body). It's an implementation detail (which does however leak into URLs as well which we should also fix eventually). See TyMethodItem.

Unrelated but existential should maybe be renamed to opaque?