to show unused block warning strictly.

```ruby
class C
  def f = nil
end

class D
  def f = yield
end

[C.new, D.new].each{|obj| obj.f{}}
```

In this case, `D#f` accepts a block. However `C#f` doesn't
accept a block. There are some cases passing a block with
`obj.f{}` where `obj` is `C` or `D`. To avoid warnings on
such cases, "unused block warning" will be warned only if
there is not same name which accepts a block.
On the above example, `C.new.f{}` doesn't show any warnings
because there is a same name `D#f` which accepts a block.

We call this default behavior as "relax mode".

`strict_unused_block` new warning category changes from
"relax mode" to "strict mode", we don't check same name
methods and `C.new.f{}` will be warned.

[Feature #15554]

* rb_warn() does not warn if $VERBOSE is nil, the "always" is wrong.
* Talk about $VERBOSE and not -W since $VERBOSE can be changed at runtime.

Followup: ac123f167a364c3d7a43eca78d564e41f6dbb91e

RB_WARN_CATEGORY_ALL_BITS is exposed in a public header, so it
makes sense for it to be updated to contain all valid bits.

Instead we introduce RB_WARN_CATEGORY_DEFAULT_BITS to list the
categories that are enabled by default.

[Feature #19538]

This new `peformance` warning category is disabled by default.
It needs to be specifically enabled via `-W:performance` or `Warning[:performance] = true`

Better document [ci skip]

Must not be a bad idea to improve documents.

Truly editorial fix for comments.  This works better with Emacs'
set-justification-full function. [ci skip]

So that it will not interfere the fast path in `Check_Type`.