rustdoc: Allow listing all items of a kind when searching for "<kind>:"

[tyilo]

For instance searching for macro: should list all macros in the crate.

[fmease]

#t-rustdoc > allow kind searches without names

[fmease]

The disposition on Zulip was "won't implement" since it'd be redundant with the "All Items" subpage.

[tyilo]

I don't see any good arguments why not to add support for this.

Why would users rather have the page show an error when searching for macro: instead of showing all macros?

[notriddle]

I'm willing to change my mind if binarycat and guillaume both think it's worth doing. I just don't want to go around adding features merely because it's easy, without any demonstrated user demand.