Tweak @GlobalScope constants documentation overall

[Mickeon]

This PR continues where #67208 left off, handling the constants part of the documentation. Pretty big stuff.

This could technically still be a draft, as I would personally prefer to first fill out all of the descriptions, then go into detail and expand over the currently outstandingly dry (I'm looking at you Error constants).
However, there's so many different ways each description could generally be formatted, and despite consulting with the documentation chat (see link for the question), no real consensus was reached, possibly out of uncertainty in the moment.
For now, I have only expanded upon ERR_BUG and ERR_HELP, and each is described in a completely different manner.
I wouldn't want to write the descriptions one way, then go back to readjust them again. I hope that is understandable.

General:

• Replace most type names with strong references (such as "integer" to "[int]");
• Made the writing style closer to how the rest of the documentation is formatted.
• Use uppercase Inspector when referring to the Inspector dock.
• Describe missing MethodFlags, Variant.Type constants.

Key:

• Add " key." to the KEY_0 to KEY_9 constants;
• Reword some MouseButton constants;
  • In a programming context, it's kind of odd hearing about the plural of "mouse". It may be just me, however.

Error:

• Reword OK constant to flow better, use printerr() over print();
• Describe where ERR_HELP is used by the engine;
• Point to the Github Issues Page in ERR_BUG.

PropertyHint:

• Mention that PROPERTY_HINT_IMAGE_COMPRESS_LOSSY and PROPERTY_HINT_IMAGE_COMPRESS_LOSSLESS are unused;
• Mention that PROPERTY_HINT_PASSWORD allows a placeholder string.
• Describe PROPERTY_HINT_OBJECT_TOO_BIG, PROPERTY_HINT_MAX.

PropertyUsageFlags:

• Describe PROPERTY_USAGE_HIGH_END_GFX, PROPERTY_USAGE_ARRAY.

[mhilbrunner]

Thank you! 🎉