Small NextState corrections

[richTrash21]

Changes:

• Removed extra bracket from NextState.fromState deprecation message.
• Removed functinality to create state instance from class using NextState.createInstance. Reasoning: NextState can't be a class.
• Updated InitialState docs:
  • Capitalized "Haxe" in "HaxeFlixel".
  • Added missing space between "FlxStates" and "could".
• Added deprecation warning for InitialState.fromType. Reasoning: docs state that it's deprecated.
• Made NextState.fromState and InitialState.fromType inlinable. Reasoning: this allows direct cast of FlxState instance to NextState and Class<FlxState> to InitialState while this functionality is supported.

[Geokureli]

I'm going to be ignoring PRs with no description from here on out, explain yourself and your reasoning
https://github.com/HaxeFlixel/flixel/wiki/Contribution-Guide#provide-a-description

[richTrash21]

I've updated the description

[Geokureli]

• Removed functinality to create state instance from class using NextState.createInstance. Reasoning: NextState can't be a class.

We should just add it. I think I planned to eventually, but didn't wanna change too much and confuse people.

• Added deprecation warning for InitialState.fromType. Reasoning: docs state that it's deprecated.

This is likely a copy/paste error on my part, let's change the doc