Improve text of “Too many changes to show” alert

This is a follow-up from #232820 (comment 402228466):

Existing alert

Proposed alert

1. This issue: Make the UI copy a bit easier to understand: Show changes are not shown For a faster experience, only **X of Y files** are shown. To see all changes, download one of the files below.
2. Make `Expand all` button more visible (#243573 - closed)
3. Clarify “Expand all” button label (#243574 - closed)