Update list of available search filters in help menu

[GuillaumeGomez]

Just realized that the help menu mentions the list of available search item type filters... but it's also very incomplete and the book says the full list is provided there.

Considering that search.js is supposed to be self-contained (for our testsuite mostly) and that we don't want to add an extra step to generate JS (so we can copy-paste it around and just test it), I'm not too sure what would be the best solution here.

Anyway, question for future us, in the meantime, this adds the missing entries in the help menu:

r? @lolbinarycat

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha, @lolbinarycat

[GuillaumeGomez]

r? @notriddle

[notriddle]

Ick, no. That is way too long for a flat list like that. Maybe, instead?

Accepted kinds are:

• struct, enum, union
• structfield, variant
• type: primitive, associatedtype
• constant: associatedconstant
• fn: method, tymethod
• trait: traitalias
• macro: attr, derive (NOTE: typePassesFilter doesn't implement this, but it should)
• import: externcrate (NOTE: typePassesFilter doesn't implement this, but it should)
• keyword, attribute
• mod
• static
• foreigntype
• generic

existential and impl exist in the code, but are anonymous, so you can't actually use them.