Clarify usage of display size

[bugadani]

Hi! Thank you for helping out with SSD1306 development! Please:

• Check that you've added documentation to any new methods
• Rebase from master if you're not already up to date
• Add or modify an example if there are changes to the public API
• Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, Changed, etc)
• Run rustfmt on the project with cargo fmt --all - CI will not pass without this step
• Check that your branch is up to date with master and that CI is passing once the PR is opened

PR description

Place some comments that (hopefully) help with the confusion around using different display sizes.

Closes #140

[jamwaffles]

Would it be a better idea to remove the default DSIZE = DisplaySize128x64 from GraphicsMode, TerminalMode, etc?

[jamwaffles]

Ah, I just saw #140. Let's go with that approach.

[bugadani]

Yes but only if you're fine with breaking changes :)

[jamwaffles]

Yep! That's fine. I need to get e-g 0.7 into this driver at some point too, so that's already breaking.

[bugadani]

@jamwaffles sorry for the noise, CI told me I'm a dummy.

[jamwaffles]

Sorry it took me so long to get to this. It all looks good to me!