Document that `yield` isn't allowed in a captured block argument

[andrykonchin]

Both "Capturing blocks" and "Block forwarding" sections mention that break and next keywords are prohibited. But AFAIK yield isn't allowed too. As well as return. It might be important because they are allowed in Ruby.

If this change is welcome, I would like to open a pull request.

https://crystal-lang.org/reference/1.18/syntax_and_semantics/capturing_blocks.html

[straight-shoota]

This specification seems actually incorrect: next is allowed inside a captured block.

def capture(&block : -> Int32)
  block
end

capture do
  next 1
end.call # => 1

The description in Capturing Blocks: break and next looks about right.

Block Forwarding needs a fix.
It's also a bit odd that half the section about non-captured blocks talks about captured blocks. 🤷

About explicitly documenting yield, that's probably a good idea. It's only allowed inside method bodies, including nested scopes that are not a captured block.

Blocks and Procs etc. discuss yield a lot, but there's no explicit definition.
It also shows in the search results that yield has no specific page or section of its own.