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`.

This adds rb_category_compile_warn in order to emit compiler warnings
with categories.  Note that Ripper currently ignores the category
for these warnings, but by default it ignores the warnings completely,
so this shouldn't matter.

Since we decided to only allowing specific warning categories,
there is no reason to have an API that accepts a general string,
as it is more error-prone.  Switch to only allowing the specific
warning categories.

As rb_category_warn{,ing} are public API, this requires making
rb_warning_category_t public API as well.

This adds the following C-API functions that can be used to emit
warnings with categories included:

```c
void rb_category_warn(const char *, const char*, ...)
void rb_category_warning(const char*, const char*, ...)
```

Internally in error.c, there is an rb_warn_category function
that will call Warning.warn with the string and the category
keyword if it doesn't have an arity of 1, and will call
Warning.warn with just the string if it has an arity of 1.
This refactors the rb_warn_deprecated{,_to_remove} functions
to use rb_warn_category.

This makes Kernel#warn accept a category keyword and pass it
to Warning.warn, so that Ruby methods can more easily emit
warnings with categories.  rb_warn_category makes sure that
the passed category is a already defined category symbol
before calling Warning.warn.

The only currently defined warning category is :deprecated,
since that is what is already used.  More categories can be
added in later commits.

To fix build failures.

Devs do not love "impl".