Small changes to introduction_from_photoshop.rst, and typo/grammar fixes on other files

I made some small grammar fixes on several files and some description changes for the introduction from Photoshop: -Eraser is described as a mode toggle because that's a more succinct way of explaining it. -It's a bit emphasized that the alpha inheritance system is overall easier to use than clipping masks, and not just a substitute. -I added Photomerge to the list of things Krita doesn't have, and included the alternative (which uses a different software) that was recommended to me when I asked on the discord server.