docs: Improve squash command description

Add an explanation for why it's called "squash".

Fixes #7661

Author: steveklabnik

cli/src/commands/squash.rs

@@ -51,6 +51,9 @@ use crate::ui::Ui;
 
 /// Move changes from a revision into another revision
 ///
+/// The name "squash" comes from the idea of combining (squashing) the changes
+/// from multiple revisions together.
+///
 /// With the `-r` option, moves the changes from the specified revision to the
 /// parent revision. Fails if there are several parent revisions (i.e., the
 /// given revision is a merge).