Remove incorrect mouseX/mouseY docs regarding WEBGL mode

[ericrav]

resolves #8002

Changes:

The docs for mouseX, mouseY, pmouseX, pmouseY incorrectly state that in WEBGL mode the number is relative to the center of the screen. In fact, is relative to the top-left corner. Removes the distinction between 2d and webgl.

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated

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Thank You!

[perminder-17]

Looks great to me! Thanks for your work.

[perminder-17]

@all-contributors please add @ericrav for doc

[allcontributors]

@perminder-17

I've put up a pull request to add @ericrav! 🎉