Improve documentation grammar and wording

[Swarkin]

This PR aims to make the documentation comments easier to read and understand.

[Swarkin]

Fixed conflicts

[Swarkin]

Fixed conflicts

[emilk]

needs cargo fmt

[Swarkin]

Hi, cargo fmt does not change anything for me.

[lucasmerlin]

@Swarkin hmm, maybe you are on an older rust version?
Checking the output of the Rust / Format + check + test check will also tell you whats wrong:

Diff in /home/runner/work/egui/egui/crates/egui/src/memory/mod.rs at line 172:
     #[cfg_attr(feature = "serde", serde(skip))]
     pub dark_style: std::sync::Arc<Style>,
 
-
     /// The default style for new [`Ui`](crate::Ui):s in light mode.
     #[cfg_attr(feature = "serde", serde(skip))]
     pub light_style: std::sync::Arc<Style>,
Diff in /home/runner/work/egui/egui/crates/egui/src/memory/mod.rs at line [18](https://github.com/emilk/egui/actions/runs/10832498741/job/30187822132?pr=5052#step:6:19)8:
     ///
     /// Default: [`crate::Theme::Dark`].
     pub fallback_theme: Theme,
-
 
     /// The current system theme, used to choose between
     /// dark and light style in case [`Self::theme_preference`] is [`ThemePreference::System`].

(you will need to remove the lines marked with -)

[Swarkin]

Done

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5052-patch-1
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.